Add flex_quickgen function for bidirectional generation divided into phases

[AntonOresten]

Allows for annealing, adding points, using recorded trajectories, tracking across phases...

To-do:

• Should use dictionaries for recording to allow different step sizes across phases. need to be careful with off-by-ones, midpoint steps etc.
• Non-linear time steps

[codecov]

Codecov Report

❌ Patch coverage is 0.60241% with 165 lines in your changes missing coverage. Please review.
✅ Project coverage is 18.25%. Comparing base (d8ea18c) to head (1af421e).
⚠️ Report is 3 commits behind head on main.

Files with missing lines	Patch %	Lines
src/flow.jl	0.89%	111 Missing ⚠️
src/reverse.jl	0.00%	40 Missing ⚠️
src/ChainStorm.jl	0.00%	14 Missing ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##             main       #6      +/-   ##
==========================================
- Coverage   25.39%   18.25%   -7.15%     
==========================================
  Files           5        4       -1     
  Lines         189      263      +74     
==========================================
  Hits           48       48              
- Misses        141      215      +74     

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