Added cd and cdtemp contexts.
Added cd and cdtemp contexts.

Added the *cd* and *cdtemp* contexts that create a context in an
existing, new, or temporary directory. Upon exiting the context, the
current directory is returned to what it was before the context and the
temporary directory is removed (with cdtemp).

csherwood-usgs/landlab,RondaStrauch/landlab,RondaStrauch/landlab,amandersillinois/landlab,landlab/landlab,SiccarPoint/landlab,ManuSchmi88/landlab,amandersillinois/landlab,landlab/landlab,ManuSchmi88/landlab,landlab/landlab,cmshobe/landlab,cmshobe/landlab,laijingtao/landlab,decvalts/landlab,ManuSchmi88/landlab,cmshobe/landlab,Carralex/landlab,Carralex/landlab,decvalts/landlab,csherwood-usgs/landlab,RondaStrauch/landlab,laijingtao/landlab,SiccarPoint/landlab,Carralex/landlab