Maintain digital volume balances also when on_underflow was used

[michaelosthege]

#102 describes a bug that was overlooked in #101 and lead to violating mass balances in the volume tracking.

This PR fixes the transfer-related worklist operations by

• Returning aspiratable volumes from Labware.remove, BaseWorklist.aspirate, EvoWorklist.evo_aspirate methods
• Adapting EvoWorklist.transfer and FluentWorklist.transfer to pass aspiratable volumes to Labware.dispense as an override such that generated commands still dispense volume, but digital liquid volume tracking is only updated with the same volume that was digitally aspirated from the source.

[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 95.78%. Comparing base (d22b766) to head (11de601).
Report is 3 commits behind head on master.

Additional details and impacted files

@@            Coverage Diff             @@
##           master     #103      +/-   ##
==========================================
+ Coverage   95.75%   95.78%   +0.02%     
==========================================
  Files          19       19              
  Lines        1202     1209       +7     
==========================================
+ Hits         1151     1158       +7     
  Misses         51       51              

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[michaelosthege]

Self-reviewed this.

Noticed that aspirating from throughs with on_underflow != "raise" can produce very confusing results in reality, depending on the order of pipetting given by liquid class settings.

The implementation will not warn about all wells at the same time, but only start warning once the iterator reaches wells for which the remaining volume in the trough is insufficient.