MAINT: LindbladCoefficientBlock blocktype branching

[rileyjmurray]

This PR has major changes to LindbladCoefficientBlock. I've taken care to produce code that is equivalent to the old code from an input-output perspective.

After this PR we'll be well-positioned to make a base class for LindbladCoefficientBlock (or just make LindbladCoefficientBlock a base class) from which new classes can inherit. I want to do this because I have an idea for a sparse Cholesky approach to reduced-order CPTP models.

The diff for this PR is kind of awful. Honestly, it's a little hopeless to track what happened. My ask is that we merge anyway once all tests pass. The approach I took to the refactor was to go function by function, preserving as much of the original implementation as possible. My first pass used an LLM for applying the refactoring pattern below. Once some tests failed after this, I did another pass that manually copy-pasted relevant codeblocks from the old perform_operation functions into the new _perform_operation_<blocktype> helper functions.

general approach

I took functions like

def perform_operation(self, *args, **kwargs):
    # some pre-branching work ...
    if self._blocktype == 'ham':
        # insert 3 to 5 lines here; call this codeblock "X"
        pass
    elif self._blocktype == 'other_diagonal':
        # insert 10 to 20 lines of code here; call this codeblock "Y"
        pass
    elif self._blocktype == 'other':
        # insert 20 to 80 lines of code here; call this codeblock "Z"
        pass
    else:
        raise ValueError('unsupported block type')
     # some post-branching work ...
     return

and refactored them to

def perform_operation(self, *args, **kwargs):
    # some pre-branching work ...
    if self._blocktype == 'ham':
        self._perform_operation_ham(*args, **kwargs)
    elif self._blocktype == 'other_diagonal':
        self._perform_operation_otherdiag(*args, **kwargs)
    elif self._blocktype == 'other':
        self._perform_operation_other(*args, **kwargs)
    else:
        raise InvalidBlockTypeError()
     # some post-branching work ...
    return

def _perform_operation_ham(self, *args, **kwargs):
    # insert codeblock "X" here
    return

def _perform_operation_otherdiag(self, *args, **kwargs):
    # insert codeblock "Y" here
    return
    
def _perform_operation_other(self, *args, **kwargs):
    # insert codeblock "Z" here
    return
    
    

[nkoskelo]

Are we sure that this ordering is always going to be the best ordering performance wise?

[rileyjmurray]

Not at all! But the point of this PR is to be minimally-invasive. In an ideal world we'd formally certify that the current refactor does not affect input-output behavior of the existing functions.