first draft of SymPyCCode op
add remove utility function
import remove
add c_headers to SymPyCCode op
add SymPyCCode.grad
protect sympy tests if no sympy
skip sympy tests with skiptest
fix typo on skiptest
small edits to SympyCCode and test

Change grad function name to "grad", not "prime"

Change y**2 test to y**3
Merge pull request #1486 from mrocklin/sympy-ccode-op

SymPyCCode op