Fix unbalanced result columns

[jkupka]

This PR fixes two issues with unbalanced result columns:

1. columns 'p_c_from_mw' and 'loading_percent' were missing in 'res_line_3ph"
2. unbalanced loss columns didn't follow naming conventions (e.g. 'p_a_l_mw' should be 'pl_a_mw'). It's already correct in the documentation but not the code:

Issue 2) is probably a breaking change if something relies on those unbalanced loss columns.