Add initial HREMD checkpointing

[SimonBoothroyd]

Description

This PR adds basic checkpointing for HREMD simulations.

Status

• Ready to go

[codecov]

Codecov Report

Attention: Patch coverage is 98.68421% with 1 lines in your changes are missing coverage. Please review.

Project coverage is 95.94%. Comparing base (b09cba6) to head (5baacf5).

Files	Patch %	Lines
femto/md/hremd.py	98.33%	1 Missing ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##             main       #6      +/-   ##
==========================================
+ Coverage   95.86%   95.94%   +0.07%     
==========================================
  Files          46       46              
  Lines        2951     3008      +57     
==========================================
+ Hits         2829     2886      +57     
  Misses        122      122              

Flag	Coverage Δ
unittests	95.94% <98.68%> (+0.07%)	⬆️

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