Trace.doit() produces Sum
MatMul.doit supports deep
add trace function
trace doctest - add use_unicode=False
Trace.doit calls obligatory doit on Sum
Identity sympifys arg
Add trivial Matrix.doit()
Trace no longer evaluates.  trace evaluates.

Also, trace no longer rewrites as Sum
Trace._eval_rewrite_as_Sum
FunctionMatrix._eval_trace = Trace._eval_rewrite_as_Sum
trace docstrings
xfail Trace rewrite as Sum test
Merge pull request #2154 from mrocklin/matrixexprs-doit

Add more `.doit` methods to Matrix Exprs