Change layer absorption calculation for BL optics

[phoebe-p]

Minor change - instead of integrating over the differentiated absorption, calculate the total absorption analytically. If you have large position spacing and high absorption, the integrated absorption calculated from the absorption profile can be quite off and sometimes > 1. Of course, this way you can get a discrepancy between the EQE (or anything calculated using the absorption profile) and the absorption per layer, but at least the total layer absorption will be correct and this way you can check if you need to use different depth spacing. This is similar to the current approach for the TMM and RCWA methods, which use the power calculated per layer explicitly rather than integrating the absorption profile.

[codecov]

Codecov Report

Merging #213 (a81dd84) into develop (29593db) will increase coverage by 0.00%.
The diff coverage is 70.58%.

@@           Coverage Diff            @@
##           develop     #213   +/-   ##
========================================
  Coverage    45.84%   45.85%           
========================================
  Files           84       84           
  Lines         9091     9100    +9     
========================================
+ Hits          4168     4173    +5     
- Misses        4923     4927    +4     

Impacted Files	Coverage Δ
solcore/optics/rcwa.py	23.88% <0.00%> (ø)
solcore/optics/beer_lambert.py	50.00% <73.33%> (+2.00%)	⬆️
solcore/optics/tmm.py	91.35% <100.00%> (ø)
solcore/optimization/differential_evolution.py	78.72% <0.00%> (-4.26%)	⬇️

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