Take care of deprecation warning in pd.concat usage

[jgieseler]

Since Pandas 2.1.0, there is a deprecation warning in (some) usages of pd.concat:

Deprecated the behavior of concat() with both len(keys) != len(objs), in a future version this will raise instead of truncating to the shorter of the two sequences (GH 43485)

This is raised e.g. for this call

solo-epd-loader/solo_epd_loader/__init__.py

Lines 826 to 828 in 7241eea

	df_epd_p = pd.concat(
	[df_p[flux_p_channels], df_p[flux_sigma_p_channels]],
	axis=1, keys=['H_Flux', 'H_Uncertainty', 'QUALITY_FLAG'])

Minimal working example:

from solo_epd_loader import epd_load
df_protons_het, df_electrons_het, energies_het = epd_load(sensor='het', level='ll', startdate=20220905, viewing='sun', autodownload=True)