Simplified PrimordialChem RHS network file (#1172)

This PR simplifies the RHS file for primordial chemistry by a significant amount, by using sympy's CSE (common subexpression elimination) process to simplify expressions for all the jacobians and d Temperature /dt. The file size has reduced from 9.8 MB to 241 kB (6k lines of code compared to 30k earlier), with a factor of 11 speed-up in compilation.

This change is proving pivotal to compiling Quokka on GPUs without memory exhaustion errors.

Tests run successfully as before. PR ready for review.