MY05 and SB06 PSDs

[erikssonpatrick]

Sice this morning this test

controlfiles/artscomponents/psd/TestPsds.arts

performs basic testing of some PSDs. I considered to include MY05 and SB06 but did not see how to apply the test on these PSDs.

Suggestion 1: Let the two psd methods output scat_species_a and scat_species_b. As I understand it, these two variables are basically set as internal varaibles. If these varaibles are output, it should be possible to test them following the pattern of the tests already in TestPsds.arts.

Suggestion 2: I think we discussed that there is a duplication of modified gamma code. Could MY05 and SB06 be changed to use the more general MG functions?