Add data structure to save and restart a simulation.

[chrisjonesBSU]

This PR adds Simulation.save_simulation_pickle and Simulation.from_simulation_pickle methods that save everything needed to save and restart a simulation (state, forces, reference values, and kwargs).

A couple notes:

1. The save_simulation_pickle method writes a binary string to the file that is checked by from_simulation_pickle before opening the pickle with pickle.load.

2. Using local snapshots, or state.get_snapshot() aren't pickleable, so we have to save a temp.gsd. file, then pickle the gsd.hoomd.Frame state instead.

[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 94.90%. Comparing base (a600b73) to head (6dbdfcc).
Report is 20 commits behind head on main.

Additional details and impacted files

@@            Coverage Diff             @@
##             main     #149      +/-   ##
==========================================
+ Coverage   94.77%   94.90%   +0.13%     
==========================================
  Files          26       26              
  Lines        1970     2021      +51     
==========================================
+ Hits         1867     1918      +51     
  Misses        103      103              

Files	Coverage Δ
flowermd/base/simulation.py	92.89% <100.00%> (+0.87%)	⬆️

[marjanalbooyeh]

This is a really good PR. Thank you for putting the effort on this Chris.
I just added one question about wall forces.