Merge pull request #1643 from Kenneth-T-Moore/bench

Group differentiable islands in their non-differentiable groups
some cleanup
fixed issue with setting a vtree subvar where another subvar is connected
eliminated the creation of an unnecessary pseudocomp if a src and dest units in a connection have different strings but are really the same unit
a little cleanup
Merge pull request #1644 from naylor-b/vtree_subvar_set

fixed issue with setting a vartree subvar when some other subvar of that vartree is already connected.
Merge pull request #1645 from naylor-b/unitstr

eliminated the creation of an unnecessary pseudocomp if a src and dest ...
a little streamlining
Merge pull request #1646 from naylor-b/algs

some modest performance improvements to the graph stuff
for bret