add fast pretty printing to sbml3
add fast pretty printing to sbml3

The flag to disable it for performance reasons is now removed, as the
pretty-printing time is much less than the time to create the xml tree.

Additionally, minidom is no longer required for pretty printing.

jerkos/cobrapy,zakandrewking/cobrapy,zakandrewking/cobrapy,jeicher/cobrapy,aebrahim/cobrapy,JuBra/cobrapy,jerkos/cobrapy,aebrahim/cobrapy,jeicher/cobrapy,JuBra/cobrapy