Pygeopack

Added ability to suppress warning about missing data file.

Setting `Model="IGRF"` uses the internal field model without any external field contributions.

Hopefully fixed some building issues on Windows 10/11

Updated Windows-DLL-related code. It appears that newer versions of Python might not be searching $PATH for DLLs, so I have added a function to scan for libstdc++*.dll and use that path.

Fixed bug in Fortran code which requires the `-fallow-argument-mismatch` argument which doesn't appear to be available on all Fortran compilers.

This release makes use of the separate [geopack](https://github.com/mattkjames7/geopack.git) library which is under development in place of the mess it used before.

This module should be installed as a source distribution so that the C++ and Fortran code is built during the installation of the package.

It also works on MacOS, surprisingly!

v1.2.0-devel
This is the first release using the `geopack` submodule. It seems to work on Linux, currently untested on Windows or Mac...