Modular fitter attempt

[settwi]

PR Description

A crack at an updated "modular" fitting interface. Notebook included demonstrating fitting a "line" of "X-ray" data.
No real tests written yet but that's on the agenda, if we want to continue with any of this.

I am curious as to what folks think!

---

Here's a copy/paste from the "prelude" to that Jupyter notebook:

Modular modeling framework example

This notebook shows how a fitting framework could be made,
in the context of issue #81 and issue #126.

sunkit-spex has (or will have?) a modular interface.
First we define the data interface.
For now it is only X-ray data but we can add more data types.

Then we define the fitting interface.
A PhotonModel represents any kind of model which represents photons which get counted.
In the future we can add different models, like IntegratedFluxModels if the
need arises; there is not a need to tie us to a base class right now.

The first example tackled here is fitting data to a line.
More complex models may be implemented however the user or developers see fit
by modifying the perform_fit method in subsequent classes.
The first class implemented is a Monte Carlo $\chi^2$ fitter.

A lot of ideas here are similar to pr #130.
I think we should pick and choose what we like from both and move from there.

[codecov-commenter]

Codecov Report

Attention: Patch coverage is 50.29586% with 84 lines in your changes are missing coverage. Please review.

❗ No coverage uploaded for pull request base (main@2deabc0). Click here to learn what that means.

Files	Patch %	Lines
sunkit_spex/fitting/fitters.py	45.58%	37 Missing ⚠️
sunkit_spex/fitting/fit_models.py	51.61%	30 Missing ⚠️
sunkit_spex/fitting/fake_models.py	28.57%	15 Missing ⚠️
sunkit_spex/fitting/spectra.py	88.88%	2 Missing ⚠️

Additional details and impacted files

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  Misses          ?     1483           
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[settwi]

@samaloney @KriSun95 @DanRyanIrish @hayesla
tear this apart plz