Solvate function doesn't support nn and np input

[VOD555]

Related to Becksteinlab/MDPOW#174
In gromacs, one can add a specific number of ions by using keywords nn and np.
But in gromacs wrapper solvate function, ions only can be added with the keywords concentration. And concentration keywords only works with some specific water models.

[jandom]

@orbeckst i've had a quick look at this and this is indeed correct – there is no support in GromacsWrapper for passing the number of positive/negative ions directly. It could be added in. Do you think it's worth it? What's the next step here? The ticket is a bit old

[orbeckst]

Probably not worth it unless someone really needs it — certainly not urgent.

[jandom]

Should we close then? Or what's the best next step?

[orbeckst]

I am ok with leaving it open — it documents that it is not available yet.

It's also not a "won't fix" — it's just not a priority.