New functions molar_density and mass_density added

[rameshputalapattu]

Hi @pw0908,
I have added new functions molar_density and mass_density and the corresponding unitful versions as well. I found that having functions which calculate densities to be quite handy based on my experience at work using other GERG2008 (fortran and C++) implementations. I have added a self contained test set for these functions.
Please see if it fits into the upstream code base and can be merged. I will be glad if these changes could get in.
Please let me know if you have any comments.

Regards
Ramesh

[pw0908]

Hi Ramesh,

Thanks a lot for this! We agree that densities would certainly a useful property to support (we have a long list of properties we want to support and just need to get around to it...).

However, you have also pointed out something we hadn't considered: supporting both mass and molar basis properties. A simple solution could be just to do as you did and simply have molar_property and mass_property functions. We do wonder if there isn't a cleaner way to do this?

One suggestion is something like:
mass_basis(property(model,p,T))
molar_basis(property(model,p,T))
If you have any suggestions, that would be great. I'll leave the pull request open until we've decided what's the best thing to do.

Cheers,

Pierre

[ypaul21]

We probably need to pass in the model struct directly to the function to allow it to access the molar mass. It would probably have to be something like

mass_basis(property, model, p, T, z)

Not sure if it’s an elegant-enough solution though.

But mass and molar densities are extremely common, and they’re some of the few intensive properties that we could support (the rest are mostly extensive). I think I’m personally fine with accepting this PR for these two properties.

[pw0908]

In that case we'll merge then. I'll move the tests into the main methods_test file later on. We can sort out the molar and mass basis issue later on.