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.
Merge branch 'master' into issue-2769
Applied super_sub to functions.
Added blank line.
Fixed merge conflict.
Fixed merge conflict.
printers > printing for sympy convention.
Fixed merge conflict.
Refactored off of master merge.
Changed to raw strings in LaTeX tests.
Minor formatting updates.
Added TODO for pretty printing tests.
Merge branch 'master' into issue-2769
Merge pull request #2772 from moorepants/issue-2769

Fixes issue #2769
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