Adding a custom parsed function

[oguevremont]

Description of the problem

• In the idea of adding a reactive physics, a way to parse functions based on any variables needed to be added. For example, when a species A reacts at a rate of -dA/dt = k*A, the source term for A depends on A itself.
• Parsed functions in deal.II didn't allow to parse functions unless they depended only on x,y,z,t.

Description of the solution

• A new custom class based on the deal.II implementation of parsed functions (using muParser) is added to Lethe.
• This class has the template parameter n_component. It requires that classes are instantiated with n_component expressions as well.
• Physics that will use this class will have the responsibility of interfacing between their variables of interest and the class ParsedFunctionCustom itself.
• When a different number of variables/expressions will be needed by a solver, this solver will have the responsibility of introducing dummy variables or expressions. This solution might change if required.

How Has This Been Tested?

• [core/custom_parsed_functions] Unit test of value and gradient functions.

Documentation

• Not at the moment, except for doxygen.

Future changes

• A reactive auxiliary physics will be implemented using this class.

[blaisb]

1-2 comments that's all. Cool work man

[blaisb]

I'm good with it. Can you get @mivaia and @hepap 's review and then I can merge

[lpsaavedra]

Great work! Excited about the reactive physics module in Lethe coming soon. I have only a few remarks regarding documentation

[hepap]

Only few suggestion for comments :)