Pass the total ion density to BeamCXPEC in BeamCXLine and add donor_metastable attribute to BeamCXPEC

[vsnever]

This PR fixes #441 and #411.

• The docstrings for the core and the openadas BeamCXPEC classes are updated to reflect that the total ion density rather than the receiver ion density is needed to evaluate the effective CX emission coefficient.
• The BeamCXLine now passes the total ion density to the BeamCXPEC.evaluate() method instead of the receiver ion density.
• The donor_metastable attribute is added to the core BeamCXPEC class.

I've added a warning to the changelog that this fix affects user results. The affected figures in the documentation have also been replaced.

Please note that tests for Python 3.9 and 3.10 will fail due to #442.

[vsnever]

Hi @Mateasek,

Thanks for review. In the comment I tried to explain why I added the donor_metastable attribute to the base BeamCXPEC and why I included the fixes for #411 and #441 in the same PR. But if you are not happy with that, I can move a fix for #411 in a separate PR.