Gflex

Major improvements (as determined by AW)
* Updated for Python 3, and now, Python 3.10+
* Updated build and configuration for current standards
* `argparse` for CLI
* Update testing and CI: Travis --> GitHub actions
* Overall professional cleaning of code by mcflugen

What's Changed
* Update repository for newer Pythons, packaging standards by mcflugen in https://github.com/awickert/gFlex/pull/44

New Contributors
* mcflugen made their first contribution in https://github.com/awickert/gFlex/pull/44

**Full Changelog**: https://github.com/awickert/gFlex/compare/v1.1.1...v1.2.0

* Updated PyPI support: twine upload, README.md
* Throw meaningful error if a nonuniform `self.Te` grid is used with the analytical solution

* Support for both Python 2 and Python 3
* Main code
* Examples
* README.md updates
* PATH updates in
* Code tests included
* Code testing on commit by Travis
* Updated on PyPI

Final Python 2 (only) release
Additional documentation added

First full release of gFlex in association with the now-accepted GMD paper, "Open-source modular solutions for flexural isostasy: gFlex v1.0", by A. D. Wickert.

This is the update is what is available now from **pypi**.
* Minor updates to the main gFlex codes
* Addition of a missing "12" in the flexural wavelength calculator in the utilities.