Ensure proper treatment of pure heating/cooling

[n-claes]

PR description

This PR ensures that pure heating and/or cooling through custom-defined functions behaves as expected.
In short:

• Ensures that heating without cooling is checked when adding non-adiabatic matrix elements; this was not the case previously.
• The default cooling curve is now "nothing" (docs have been updated), to prevent unnecessary table initialisations and interpolations for cases in which a custom cooling function is provided.
• Prevents setting custom functions when the corresponding physical effect is not enabled.
• Throw an error if thermal balance is enforced but heating is disabled.
• Sanity checks are now in place when treating forced thermal balance.
• New unit tests for these particular cases are now in place.

Bugfixes

Legolas

• Fixes Pure heating/cooling does not work as expected #144

[codecov]

Codecov Report

Merging #145 (ecfa5a3) into develop (2a94ba9) will increase coverage by 0.01%.
The diff coverage is 83.33%.

Additional details and impacted files

@@             Coverage Diff             @@
##           develop     #145      +/-   ##
===========================================
+ Coverage    90.78%   90.79%   +0.01%     
===========================================
  Files          159      159              
  Lines         9050     9086      +36     
===========================================
+ Hits          8216     8250      +34     
- Misses         834      836       +2     

Flag	Coverage Δ
legolas	93.11% <83.33%> (+<0.01%)	⬆️
pylbo	86.70% <ø> (ø)

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