Xarray-simlab 0.4 related updates

[benbovy]

• [x] define static variables (grid processes, maybe elsewhere like stratigraphic horizons) #14
• [x] index variables (grid processes, maybe elsewhere like stratigraphic horizons) #14
• [x] use multiple groups for some variables #15
• [x] object variables? 04ea2d530b9fed28691accc4b23702974a20b0c6
• [x] fix compatibility with dask.distributed -> can't pickle fortran objects #16
  • Avoid using fastscapelib_fortran.fastscapecontext directly -> declare instead all variables that we need (tedious, verbose but at least those are serializable).

Nice to have:

• [x] default values (e.g., SPL "n" and "m" exponents, maybe other parameters) #17
• [x] values validators (ranges, choices, etc.) #17