Depletion module code improvements

[yardasol]

Description

This issue contains several small proposals to improve source code readability in the depletion module.

1. Some key internal functions in openmc.deplete do not have docstrings:

• Integrator._timed_deplete()
• SIIntegrator._get_bos_data_from_operator()
• Integrator._get_start_data()
  Docstrings should be added to these functions.

2. Some classes/functions could benefit from different internal variable names:

• Integrator._i_res. Maybe step_index
• p used instead of source_rate in SIIntegrator.integrate()
• x, n, bos_conc used interchangeably for the nuclide concentration in Integrator.integrate(), SIIntegrator.integrate(), Integrator._get_bos_data_from_operator(), etc. This also applies to n_list. We should pick one name and use it consistently across the entire codebase. I suggest concentration
• res, res_list in the same functions as the above item. I suggest just expanding this to result.
• n in deplete()
• func in deplete() to something more descriptive (this function solves the depletion matrix).
  I'd love to hear other's opinions on the naming scheme.

3. Rename the __call__() class methods to something more readable

• Integrator.__call__() to something like Integrator.integrate_single_step()
• IPFCramSolver.__call__() to something like IPFCramSolver.solve()
• TransportOperator.__call__() to something like TransportOperator.simulate()

4. Collapse DepSystemSolver into IPFCramSolver

• DepSystemSolver is an abstract class with a single abstract method, and only has a single child class (IPFCramSolver). I propose removing DepSystemSolver unless the intention is to build it out to enable other kinds of depletion matrix solvers.

5. Remove the matric_func parameter in deplete()

• My understanding is that we exclusively use the Chain.form_matrix() function for this. Are there cases where we want to allow a user to write their own function to build the matrix?

Alternatives

Compatibility

This may require some changes to docstrings depending on what action is taken.