Chaste-codegen

- Added support for Sundials 6.0.0 which requires a newly introduced SUNContext object. This version of chaste_codegen now generates code that will work with Sundials 6.0, but is backwards compatible with previous Sundials versions and the 2021 release of Chaste.

- Fixed an issue with unused variables appearing in common terms in jacobians.
- Fixes a bug printing large integers, in some environments: Large numbers such as 8.034023767017109e+27 which were actually ints would be printed as an the int 8034023767017108950029959168 and then the C++ compiler would complain as that's more than the maximum int value. This has been fixed by only printing ints as ints if they are `MIN_INT < number < MAX_INT` else we print them as a float (and we get the sceintific notation).
- Fixed tests to pass with sympy 1.10 and required latest cellmlmanip, which also works with sympy1.10. Updated sympy requirement to be >=1.9, < 1.11

- Corrected a type in the generated output for `--rush-larsen-c`
- Updated the help text to no longer imply that CellML files can be converted from remote URIs (a local file is required)
- Fixed required sympy version to be < 1.9 since the jacobean generation has in sympy1.10 changed and makes chaste_codegen tests fail

- This version drops python 3.5 support. The reason for this is python 3.5 is end of life and the chase project will soon be dropping support.
- Corrected Rush-Larsen output for `--rush-larsen-c`

- Performance upgrade for `--rush-larsen` using caching on linearity checking.

- Corrected a typo in command line argument `--skip-ingularity-fixes` renaming it to `--skip-singularity-fixes`