Add RSR and VE equation to hydromt stats

[hboisgon]

Kind of request

Adding new functionality

Enhancement Description

Add a new efficiency RSR and VE (volumetric efficiency) equations to hydromt stats.skills

Use case

In addition to existing ones like NSE, KGE. This is a more understandable version of RMSE which we do support already.

Additional Context

Paper can be found here: https://www.researchgate.net/publication/289540757_Hydrologic_and_Water_Quality_Models_Performance_Measures_and_Evaluation_Criteria

RSR = RMSE / STDev(Obs)

VE

[hboisgon]

Paper ref for VE: https://www.sciencedirect.com/science/article/abs/pii/S0022169421007228 Other papers to double check formulas:

• https://www.mdpi.com/2306-5338/1/1/20/htm#:~:text=The%20RMSE%2Dobservations%20standard%20deviation,better%20the%20model%20simulation%20performance.
• https://juniperpublishers.com/ijesnr/pdf/IJESNR.MS.ID.556003.Appendix%20B.pdf