fix docstring tests in ode.py

- remove broken separable-reduced test
fix constantsimp tests
fix test_ode for new constants
minor test_pde fix
fix arbitary-order linear ode solvers constants

- now, use a list of symbols rather than a generator
refactor get_new_constants procedure
fix constant numbering of ordinary series tests
allow 'exclude' keyword for numbered_constants
terms_gcd no longer converts Eq to Expr when factoring
allow numbered_symbols to exclude symbols
spelling changes
review-related minor modifications
watch out for negative args of log in evalf
reference issue 5080 on LambertW simplification
clean test_ode with corrected LambertW examples
Remove an unnecessary blank line
Add gamma function to CCodePrinter
Implement loggamma function for integral arguments
Implement loggamma function for half-integral arguments
Implement loggamma function for infinite arguments