HUF package support

[matejgedeon]

I like to have a HUF package support in flopy3, so I downloaded your code and added it. Now, my flopy reads and writes the HUF package file, but then I realized that I am missing zone and mlt files, which are of course not written as you fill up all parameter defined variables to native arrays for all packages. However, this approach is not possible for HUF as there is no way to write a HUF file without the use of parameters. What do you think would the best way to solve this issue:

1. Change HUF to LPF package upon reading the HUF input file. This would require the smallest change of the flopy code, but one would use all the hydrogeologic model definition which is not very practical for some applications, like parameter estimation;
2. Add parameter definition support. The user could decide if he wants to write the input files using parameters or native arrays. This would make flopy flexible, but maybe complicated;
3. Add parameter support only for the HUF package. So pval is not written, but mlt and zone files are written when HUF is present.

[langevin-usgs]

Our preference at this time is to not support HUF as it requires parameters and can be difficult to use effectively.

[Olsthoorn]

What a pity, Chris, as the HUF package seems the only option to use elevation-dependent specific yield and elevation-dependent conductivity in water-table problems, problems for which standard MODFLOW, and, yes, even USG still fails if one implements it using multiple layers. With the HUF a single layer would suffice.