Fix some style idioms in physics.mechanics
Fix some style idioms in physics.mechanics

There were a few calls of the type `f.__call__(arg)` instead of `f(arg)` and a
few complicated loops over `range()` that were easily converted to much shorter
and readable `reduce` calls and list comprehention.

It also fixed somehow a bug with a recent change in the `__call__` method of
Expr.

aktech/sympy,lidavidm/sympy,aktech/sympy,VaibhavAgarwalVA/sympy,oliverlee/sympy,wyom/sympy,sampadsaha5/sympy,yashsharan/sympy,oliverlee/sympy,souravsingh/sympy,wyom/sympy,sunny94/temp,madan96/sympy,pbrady/sympy,Titan-C/sympy,Gadal/sympy,sahilshekhawat/sympy,aktech/sympy,sahmed95/sympy,dqnykamp/sympy,jaimahajan1997/sympy,AunShiLord/sympy,dqnykamp/sympy,Arafatk/sympy,liangjiaxing/sympy,debugger22/sympy,wyom/sympy,kmacinnis/sympy,sunny94/temp,AkademieOlympia/sympy,kaichogami/sympy,kevalds51/sympy,kumarkrishna/sympy,lidavidm/sympy,shipci/sympy,sampadsaha5/sympy,chaffra/sympy,chaffra/sympy,souravsingh/sympy,AunShiLord/sympy,kumarkrishna/sympy,garvitr/sympy,wanglongqi/sympy,bukzor/sympy,mcdaniel67/sympy,Mitchkoens/sympy,atsao72/sympy,pbrady/sympy,skidzo/sympy,AkademieOlympia/sympy,mafiya69/sympy,rahuldan/sympy,drufat/sympy,meghana1995/sympy,vipulroxx/sympy,ahhda/sympy,Davidjohnwilson/sympy,farhaanbukhsh/sympy,maniteja123/sympy,farhaanbukhsh/sympy,Mitchkoens/sympy,Arafatk/sympy,grevutiu-gabriel/sympy,atreyv/sympy,Gadal/sympy,AunShiLord/sympy,garvitr/sympy,sunny94/temp,Vishluck/sympy,hargup/sympy,grevutiu-gabriel/sympy,saurabhjn76/sympy,yashsharan/sympy,postvakje/sympy,beni55/sympy,jbbskinny/sympy,emon10005/sympy,mafiya69/sympy,hrashk/sympy,Davidjohnwilson/sympy,madan96/sympy,ChristinaZografou/sympy,mcdaniel67/sympy,drufat/sympy,vipulroxx/sympy,sahilshekhawat/sympy,ga7g08/sympy,Curious72/sympy,chaffra/sympy,Sumith1896/sympy,skirpichev/omg,kaichogami/sympy,shipci/sympy,debugger22/sympy,jbbskinny/sympy,saurabhjn76/sympy,kaushik94/sympy,beni55/sympy,yukoba/sympy,Shaswat27/sympy,farhaanbukhsh/sympy,amitjamadagni/sympy,kmacinnis/sympy,wanglongqi/sympy,lindsayad/sympy,Arafatk/sympy,VaibhavAgarwalVA/sympy,shikil/sympy,MechCoder/sympy,Sumith1896/sympy,Titan-C/sympy,jbbskinny/sympy,atreyv/sympy,toolforger/sympy,rahuldan/sympy,sampadsaha5/sympy,kaichogami/sympy,iamutkarshtiwari/sympy,maniteja123/sympy,Designist/sympy,wanglongqi/sympy,cccfran/sympy,ga7g08/sympy,Davidjohnwilson/sympy,lindsayad/sympy,moble/sympy,maniteja123/sympy,MechCoder/sympy,debugger22/sympy,souravsingh/sympy,emon10005/sympy,atreyv/sympy,kaushik94/sympy,moble/sympy,jamesblunt/sympy,vipulroxx/sympy,Curious72/sympy,abhiii5459/sympy,jerli/sympy,Shaswat27/sympy,cccfran/sympy,liangjiaxing/sympy,pandeyadarsh/sympy,iamutkarshtiwari/sympy,hargup/sympy,asm666/sympy,hrashk/sympy,garvitr/sympy,sahmed95/sympy,amitjamadagni/sympy,ga7g08/sympy,yukoba/sympy,diofant/diofant,lidavidm/sympy,liangjiaxing/sympy,rahuldan/sympy,hrashk/sympy,Gadal/sympy,ahhda/sympy,Titan-C/sympy,skidzo/sympy,toolforger/sympy,ahhda/sympy,Mitchkoens/sympy,jerli/sympy,Designist/sympy,beni55/sympy,VaibhavAgarwalVA/sympy,dqnykamp/sympy,pandeyadarsh/sympy,MridulS/sympy,cswiercz/sympy,atsao72/sympy,grevutiu-gabriel/sympy,iamutkarshtiwari/sympy,MechCoder/sympy,emon10005/sympy,kaushik94/sympy,jaimahajan1997/sympy,skidzo/sympy,shipci/sympy,Designist/sympy,cccfran/sympy,MridulS/sympy,pbrady/sympy,flacjacket/sympy,Shaswat27/sympy,ChristinaZografou/sympy,yukoba/sympy,postvakje/sympy,MridulS/sympy,abloomston/sympy,Curious72/sympy,kevalds51/sympy,Vishluck/sympy,moble/sympy,mcdaniel67/sympy,bukzor/sympy,abloomston/sympy,Vishluck/sympy,oliverlee/sympy,atsao72/sympy,kmacinnis/sympy,lindsayad/sympy,madan96/sympy,AkademieOlympia/sympy,mafiya69/sympy,yashsharan/sympy,Sumith1896/sympy,asm666/sympy,kevalds51/sympy,sahilshekhawat/sympy,asm666/sympy,pandeyadarsh/sympy,sahmed95/sympy,abhiii5459/sympy,saurabhjn76/sympy,meghana1995/sympy,kumarkrishna/sympy,jamesblunt/sympy,cswiercz/sympy,hargup/sympy,drufat/sympy,bukzor/sympy,toolforger/sympy,abhiii5459/sympy,cswiercz/sympy,jaimahajan1997/sympy,abloomston/sympy,srjoglekar246/sympy,meghana1995/sympy,jamesblunt/sympy,shikil/sympy,jerli/sympy,shikil/sympy,postvakje/sympy,ChristinaZografou/sympy