Number solver

[sudarshanv01]

Changed and added files

This pull request contains the numbers solver, details of which may be found in in this manuscript. In brief, this merge request contains the following features:

• catmap.solvers.solver_base.NewtonRootNumbers: which is a modification of the Newton-root code to deal with numbers instead of coverages
• catmap.solvers.numbers_solver: which houses somewhat stand-alone classes that are responsible for converting numbers into coverages, as well as providing some important derivatives needed in the rest of the code.
• Modifications to catmap.solvers.steady_state_solver to provide "hooks" where numbers_solver classes can be used instead of the coverage solver.
• Minor modifications in thermodynamics and min_resid_mapper to facilitate the right quantities being passed around.
• Tests, which can be run using pytest are stored in the tests directory. I only test the classes and methods that I implement in the solver routines in this nerge request, no further testing of the code is done.

Practical details

The easiest way to activate the numbers solver is to use the following keys:
use_numbers_solver = True

Optional flags

A few different variations of the solver are possible. fix_x_star: bool fixed x_star if set to True and numbers_type: str = squared | exponential decides if the conversion from numbers to coverage is done by a squared function or an exponential function.