Refactor options manager

[connorjward]

Key changes:

• Allow users to set command line options for autogenerated prefixes, this will throw a warning eventually.
• Handle most options work at the point that the inserted_options context manager is used. I don't think we should be handling things in the constructor because we then can't adapt to modifications to the Options() object.
• Don't overwrite options set on the command line, and don't drop them after first use.

Closes #37

The MFE in the issue is fixed by this:

from firedrake import *
import pytest


@pytest.mark.parametrize("options_prefix", (None, "", "pippo_"))
def test_change_command_line(options_prefix):
    mesh = UnitSquareMesh(2, 2)
    V = FunctionSpace(mesh, "CG", 1)

    u = Function(V)
    v = TestFunction(V)

    F = inner(u, v) * dx - conj(v) * dx

    problem = NonlinearVariationalProblem(F, u)
    
    # suppose users pass options by command line or via environment variable
    # here we mimic this by inserting the option directly in PETSc database
    # PETSc behavior: -some_option_whatever equivalent to -some_option_<n>_whatever for all n integers
    opts = PETSc.Options(options_prefix if options_prefix is not None else "firedrake_")
    opts["ksp_type"] = "minres"
    solver = NonlinearVariationalSolver(problem, solver_parameters={"ksp_type": "cg"}, options_prefix=options_prefix)
    opts_not_removed = "ksp_type" in opts and opts["ksp_type"] == "minres"
    # remove our option so that it won't conflict with all other tests
    del opts["ksp_type"]
    assert solver.snes.ksp.getType() == "minres" and opts_not_removed

[connorjward]

@JHopeCollins this is only a rough first attempt but I want to check with you on the approach. The main thing I'm doing is moving the options logic into the inserted_options context manager/method instead of __init__. I think that this makes sense because users should be allowed to noodle with PETSc.Options() themselves. Currently the options are baked in when we construct the OptionsManager.