Allow setting volatile abundances as both metallicity or concentration

[nichollsh]

Following #292, this change extends the methods for setting the planet's volatile inventory.

It is now possible to set the volatiles as follows:

• Hydrogen: equivalent Earth oceans (H_oceans) or relative to mantle mass (H_ppmw)
• Carbon: as metallicity (CH_ratio) or relative to mantle mass (C_ppmw)
• Nitrogen: as metallicity (NH_ratio) or relative to mantle mass (N_ppmw)
• Sulfur: as metallicity (SH_ratio) or relative to mantle mass (S_ppmw)

The code prints an error and exits if more than one option is chosen for the same volatile.

Closes #296

[lsoucasse]

I find this a little confusing. It feels like the user has to fill in both methods for each element in the config file.

What if we set a default value of zero for all these entries? Then we only write in the config file the non-zeros parameters. The checks you implemented make sure that we do not overprescribe.

[nichollsh]

Fair point - it could be confusing until they run the code and get an error. However, I do think that we should include in the configuration files all the possible parameters so that the user knows what is available. As we discussed before, it's likely that they will not immediately check the documentation*.

We could add default values of zero as you describe and then comment-out (but leave in the file) the unused parameters in the configuration. That way the user knows that they are available, but they aren't expected to actually provide both X_ppmw and XH_ratio at the same time. @lsoucasse what do you think of this?

* maybe we could add something in the code that prompts the user to check the docs if something fails?

[lsoucasse]

Fair point - it could be confusing until they run the code and get an error. However, I do think that we should include in the configuration files all the possible parameters so that the user knows what is available. As we discussed before, it's likely that they will not immediately check the documentation*.

We could add default values of zero as you describe and then comment-out (but leave in the file) the unused parameters in the configuration. That way the user knows that they are available, but they aren't expected to actually provide both X_ppmw and XH_ratio at the same time. @lsoucasse what do you think of this?

• maybe we could add something in the code that prompts the user to check the docs if something fails?

Commented the unused options is a good idea so that indeed the user know there is an alternative without looking at the doc. I am happy with this solution.