Fixes issue #2769

The translate function in the latex printing module was not being called on
single variables in vector expressions. This is now implemented in the
mechanics latex printer so that variables show up correctly in Latex printing.
Note that gamma is reserved for gamma functions in SymPy and is not considered
a valid variable name for simple translation to its latex form.
Spelling and expansion of note.
Added one more test.
Added test to exhibit Matrices inablity to print greek letters.

This seems to happen with Greek letters are used as function names are elements
of Matrices.
cse hanlding of IndexedBase
Added test for cse of Indexed instances
Merge branch 'master' into issue-2769
Applied super_sub to functions.
Added blank line.
Revert "cse hanlding of IndexedBase"

This reverts commit a574d35bd7916ba7bb30eff731984e6d6afd493e.
Added NotIterable mixin class, added same to default exclude tuple in iterable()
Added NotIterable to parents of tensor.IndexedBase (enables cse)
Tidying up of test_cse.
Added tests for iterable() on Indexed and MatrixSymbol, added xfail test of cse on matrixexpr
Extended docstring of iterable() and NotIterable
Reverted NotIterable mixin of MatrixExpr
Reverted test for iterable(MatrixExpr())
Made DeferredVector NotIterable and added test for it
Removed lint print statement in test
Fixed merge conflict.