Omnifit

- Python 3 support! At least 3.8 should work, but probably also 3.9.
- Fix several instances of deprecated code, including replacing astropy_helpers with the newer package template format
- Builds are no longer handled by travis, but by github workflows

- Properly parsing requirements on installation now (issue 15)
- Made fitting compatible with lmfit versions higher than 0.9 (issue 22)
- Consequently this made omnifit incompatible with lmfit versions lower than 0.9, so dropped support for those
- Made spectrum convolution work properly with astropy 1.1
- Updated full fitting example in documentation to reflect the new format being used for complex refractive indices

- Kramer-Kronig relation implementation added to utils
- Example for using the KK relation added to documentation
- Tests for KK relation
- Added pip-requirements file
- Fixed issue 16 of spectrum convolution breaking units

- Fixed issue 12 of interpolation breaking units
- Fixed example in documentation and made it more easy to test

This version adds pypi support.