Chaste-codegen

- Fixed issue with backward euler opt models where unused state variables appeard in ComputeResidual
- Fixed memory leek on generated models within lookup table interpolation on nan/inf values in singularities.
- Removed chaste warnings from backward euler template for cases where inf/nan would end up in the table. Intead an error is thrown when multiple (more than 2) such warnings happen.

- Added an automatic fix for removable singularities in GHK equations (which can be switched off with --skip-ingularity-fixes).
The process looks for equations of any of the following forms, where U is a function of V:
- `U / (exp(U) - 1.0)`
- `U / (1.0 - exp(U))`
- `(exp(U) - 1.0) / U`
- `(1.0 - exp(U)) / U`
It replaces these with a piecewise 1e-7 either side of U==0 drawing a stright line in the region.
For example `(V + 5)/(exp(V + 5) - 1)` becomes `((fabs(-V - 5.0000000000000000) < fabs(-4.9999999000000000 / 2 - -5.0000001000000000 / 2)) ? ((-5.0000001000000000 + 5.0) / (-1.0 + exp(-5.0000001000000000 + 5.0)) + (--5.0000001000000000 + V) * ((-4.9999999000000000 + 5.0) / (-1.0 + exp(-4.9999999000000000 + 5.0)) - (-5.0000001000000000 + 5.0) / (-1.0 + exp(-5.0000001000000000 + 5.0))) / (--5.0000001000000000 - 4.9999999000000000)) : ((5.0 + V) / (-1.0 + exp(5.0 + V))))`

See for more details appendix B in: Johnstone, R. H. (2018). Uncertainty characterisation in action potential modelling for cardiac drug safety [PhD thesis]. University of Oxford. https://ora.ox.ac.uk/objects/uuid:0a28829c-828d-4641-bfb0-11193ef47195
- For lookup tables prevented expressions of the form 1 / A from being added, instead adding A. 1 / A type expressions were causing issues, when A is 0. While many cases have a piecewise to prevent hissing this case, lookup table interpolation might cause issues.
- Updated the way BackwardEuler models are calculated, to allow the jacobian to be taken into consideration for lookup tables.
- Fixed a bug with BackwardEuler models where jacobian common term equations (e.g. var_x0) ended up in lookup tables.

- Fixed sympy deprecation warning when using sympy 1.7 and bumped cellmlmanip recuirement up to ensure sympy 1.7 compatibility
- Improved support for secondary trigonometric functions such as sec and acoth.
- When used with Cellmlmanip version 0.2.2+ an improved printing of devisions is used. For example `1 / (1/cos(x))` now gets rendered as `cos(x)` whereas previously it would be `1 / 1 / cos(x)` giving the incorrect result. An side effcet of the change is that powers of formulas get extra brackets e.g. `pow((1 / x), 2)`.
- chaste_codegen uses placeholder functions for some common maths functions like exp, in order to delay evaluation till the point where the code is written. For programmers using chaste_codegen as a library, there now is a function called `subs_math_func_placeholders` which can be used on any sympy expression to substitute these placeholders.
E.g.

>> expr
2.0 * exp_(V)
>> subs_math_func_placeholders(expr)
2.0 * exp(V)

- Added an additional error messages if cellml files can't be loaded and a warning if a lookup table is specified for a tag not present in the model.

- Error messages have been improved, especially for errors caused by invalid or missing metadata.

- Corrected usage mentioned in the readme

- Now implements lookup tables.
- Multiplication equations x * y * y now have 1.0 terms removed in a way that works more generically.