NLTE fixes - discussion needed before merge - relates to #109

summary:

- bug fix in plasma_array, wrong indentation on line 587
  (calculate_nlte_level_populations)
- simplification of nlte rates - treatment of stimulated emission no
  longer handled as negative absorption (correction factor removed
  from calculate_nlte_level_populations / reformulated terms)


LTE check made using this version He I and He II, but reproducible test case needed - this needs to be discussed/explained how to insert set up.

@wkerzendorf look at the change proposed in the formulation of the rate equations. Does this look problematic? This relates to the suggestion in #109

- suggestion is to replace use of the stimulated correction factor in the rate equations (that is treating stimulated emission as negative recombination) with direct use of stimulated emission as term. Seems simpler to follow and behaves better in the LTE tests

- we need to decide if this is ok, test results compared to previous NLTE calculations (rerun one or more from paper) and put in place some mechanism to make sure we have a solution to these equations as one of our tests (I can propose what that test would be but need guidance on how to implement it)

Do not recommend merging this until it has been discussed fully.