Fix issue with movestat and multi-month seasons

[mathomp4]

As found by @michellefrazer, she had plots with names like:

GCMv12rc12_GCMv12rc12_stats2_hght_rmscmp_AMPLITUDE_EUR_z_JUL-AUG-SEP-OCT

But when movestat ran, on polar things were in folders like OCT.

The issue is that the script assumed things would always be in three-letter "seasons". This PR makes a change so that the "season" is everything after the last underscore. So instead of OCT it is JUL-AUG-SEP-OCT.

NOTE: This has been tested by @michellefrazer, but it should probably be tested in experiments with a single-month "season".

[michellefrazer]

@mathomp4 I'm having a new related issue where movestat is interpreting seasons wrong again.

With names like GenCast-FP-1.0deg-e1_GenCast-FP-1.0deg-e1_stats_uwnd_corcmp_GLO_z_MAY.gif, the season is interpreted as P-1.
With names like GenCast_FP_1.0deg_e1_GenCast_FP_1.0deg_e1_stats_uwnd_corcmp_GLO_z_MAY.gif, the season is interpreted as P_1.

This PR was supposed to make the "season" everything after the last underscore, but it appears that somehow it is actually looking for a 3-character string before the first period.

I can of course avoid periods in my experiment names in future, but I'm wondering if the code should be changed to focus on the underscore instead of the period? Or look for the last period instead of the first one?

Thanks!

[mathomp4]

@michellefrazer After yelling at ChatGPT for a while, I might have something that works:

❯ echo "GenCast-FP-1.0deg-e1_GenCast-FP-1.0deg-e1_stats_uwnd_corcmp_GLO_z_MAY.gif" | sed 's/.*_\([^_][^_]*\)\.[^.][^.]*$/\1/'
MAY
❯ echo "GenCast_FP_1.0deg_e1_GenCast_FP_1.0deg_e1_stats_uwnd_corcmp_GLO_z_MAY.gif" | sed 's/.*_\([^_][^_]*\)\.[^.][^.]*$/\1/'
MAY
❯ echo "GCMv12rc12_GCMv12rc12_stats2_hght_rmscmp_AMPLITUDE_EUR_z_JUL-AUG-SEP-OCT.gif" | sed 's/.*_\([^_][^_]*\)\.[^.][^.]*$/\1/'
JUL-AUG-SEP-OCT

Can you look at #141 ?