Avoid recalculating similar metrics

[s-scherrer]

pytesmo/src/pytesmo/validation_framework/metric_calculators.py

Lines 763 to 773 in 4070f2e

	rmsd = df_metrics.rmsd(data)
	rmsd_dict = rmsd._asdict()
	# calculate MSE
	mse, mse_corr, mse_bias, mse_var = df_metrics.mse(data)
	mse_dict, mse_corr_dict, mse_bias_dict, mse_var_dict = \
	mse._asdict(), mse_corr._asdict(), mse_bias._asdict(), mse_var._asdict()
	# calculate RSS
	rss = df_metrics.RSS(data)
	rss_dict = rss._asdict()

Currently metrics which are very similar and could be converted into each other easily are often recalculated, e.g. in rmsd, RSS and mse in the above example (where rmsd = sqrt(mse) and RSS = n * mse.

[s-scherrer]

Closed via #219