Fix SymEngine code generation. Closes #24.

• replaces mat / scalar with mat * scalar**-1.
• replace list argument to subs with a dict

(Although there are pending changes to SymEngine for the above two issues, they aren't released in a version yet.)

• Removed deep as a kwarg to expand(). It doesn't do what we want anyway.

Some test fixes:

• Use SUMPY_FORCE_SYMBOLIC_BACKEND to force usage of SymEngine. Change the behavior so that it errors out if it doesn't find SymEngine.

• Fix the SymEngine channel in the conda environment.