Resolve compilation issue; 3 ET options

[jaideep777]

In this PR I have:

1. Removed nested functions, so code should compile on mac
2. Added 3 ET options via params_siml:
   • use_gs - whether to use pmodel/phydro derived stomatal conductance in ET calcs. If false, SPLASH-based Priestly Taylor ET will be used.
   • use_pml - when use_gs is true, this determines if PML formulation is used for ET calcs. If false, diffusion equation is used. Note that currently PM is not implemented, so always set this to false.
3. Removed garbage files from vignettes. sensititivity_analysis.Rmd doesn't compile, I don't know why.
4. Repo still uses external data for the pmodel_use_newdata.Rmd vignette. I didn't have time to fix this at the moment.

The three ET options are demonstrated by pmodel_use_newdata.Rmd vignette

[jaideep777]

Now I have also added the new format demo data to the data folder, so the vignette pmodel_use_newdata.Rmd should compile without relying on external data

[stineb]

Thanks a lot for this gargantuan effort @jaideep777!