When setting magnitude picks: Only recalculate corresponding station magnitude, reuse existing other station magnitudes

[megies]

When setting a lot of station magnitudes, during setting new ones or repicking existing amplitudes, all station magnitudes are recalculated each time. This gets very annoying and laggy when many amplitude picks are set. This needs to be fixed, such that only the currently edited station magnitude (and the network magnitude) is recalculated.