[WIP] - Simulation updates

[TomDonoghue]

This is an update through the new simulation functionality added in #329

Most of the updates are cleaning up and improving consistency across the sim module. This includes moving some functions around and re-organization sim sub-modules. A notable update is the adding simulation I/O functionality, so that simulations can be saved and loaded.

[codecov[bot]]

Codecov Report

Attention: Patch coverage is 98.26690% with 10 lines in your changes missing coverage. Please review.

Project coverage is 98.24%. Comparing base (e494f93) to head (e1f186f). Report is 36 commits behind head on main.

Files with missing lines	Patch %	Lines
neurodsp/sim/generators.py	83.33%	3 Missing :warning:
neurodsp/sim/io.py	95.94%	3 Missing :warning:
neurodsp/sim/params.py	98.07%	1 Missing :warning:
neurodsp/sim/signals.py	98.30%	1 Missing :warning:
neurodsp/sim/utils.py	0.00%	1 Missing :warning:
neurodsp/utils/checks.py	50.00%	1 Missing :warning:

Additional details and impacted files

```diff @@ Coverage Diff @@ ## main #336 +/- ## ========================================== + Coverage 98.02% 98.24% +0.22% ========================================== Files 110 115 +5 Lines 3995 4327 +332 ========================================== + Hits 3916 4251 +335 + Misses 79 76 -3 ```

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[TomDonoghue]

@ryanhammonds - thanks for checking in on this!

I should have totally had a look what was available in ndspflow - it totally slipped my mind to do so! At some point we should work through what is available where and consider consolidating / linking out to other implementations etc.

Also agreed on adding parallel support - I think that would be a great extension!

For now, I'll merge this in!