Input file parsing

[wpbonelli]

We have discussed/prototyped using lark to decode MF6 input files into Python containers and primitives (before passing these to cattrs for structuring).

The prototype is currently (mostly) functional. But we still need to determine how performance compares to manual decoding. This will involve benchmarking/profiling the lark prototype against flopy3. If it becomes evident that flopy3 can be optimized in fairly straightforward ways, maybe it will be useful to add a minimal hand-written decoder prototype here for better comparison.

This is related to the performance testing mentioned in #49 (comment), but the focus areas are slightly different:

• here we are only concerned with load performance, and can ignore write performance
• in the other issue, we are most concerned with IO for data structures (arrays/tables/lists) specifically, where here we consider the full contents of input files, including internal and/or external data