Add LFM (Merkin et al 2010) ionosphere solver test.

[ursg]

This adds a small test from the Merkin et al (2010) paper, where eq 13 specifies a ionosphere FAC test case with an analytic solution.

Also gives the miniApp's makefile a much more flexible multipole test up to arbitrary orders.

This is, of course, then being used to produce plots for the ionosphere paper.

[ursg]

This now automatically runs the ionosphere tests as a CI stage (on Github cloud runners) and produces an "ionosphereTestResults" artefact that contains plot output pictures. Now if only those could be somehow displayed as a check result, like the testpackage results...