Add something to view model decision options without diving into `mDecisions.f90`

[wknoben]

Issue: figuring out the exact spelling for model decisions is a bit cumbersome right now (requires knowing that these are found in mDecision.f90 for one, and then opening the file and finding the correct lines).

Some command line argument like:

> summa.exe --decision-options hc_profile
Current options for hc_profile (choice of hydraulic conductivity profile):
- constant: constant hydraulic conductivity with depth
- pow_prof: power-law profile

could be helpful. Unsure how feasible this is though.

Alternatively, clarifying various error messages could also help. E.g.:

FATAL ERROR: summa_paramSetup/mDecisions/power-law hydraulic conductivity profile must be selected 
when using topmodel baseflow option

would be better as:

FATAL ERROR: summa_paramSetup/mDecisions/power-law hydraulic conductivity profile must be selected 
when using topmodel baseflow option. Set 'hc_profile' to 'pow_prof' in model decisions input file.

This would also help users fix "unknown decision" errors more quickly. E.g.

FATAL ERROR: summa_paramSetup/mDecisions/unknown hydraulic conductivity profile [option=power-law]

would be easier as:

FATAL ERROR: summa_paramSetup/mDecisions/unknown hydraulic conductivity profile [current option=power-law; available = constant, pow_prof]

but this would require keeping the list of decisions updated in all these different statements as well. Reading this programmatically from somewhere would make maintenance easier.