Progressively enable meaningful type checking by setting check_untyped_defs = true

Although mypy runs on the whole code base, most of the library code is not meaningfully checked, as in the current configuration untyped functions are ignored.

A first step could be to include in the env type stubs for 3rd party libs like pandas, lxml, scipy, for more accurate checks.

Over time we could opt-in certain modules to more meaningful type checking with e.g.:

# pyproject.toml
[[tool.mypy.overrides]]
module = [
    "resqpy.crs",
    "resqpy.time_series._any_time_series",
    # add here as each is cleaned
]
check_untyped_defs = true