Solo-epd-loader

<!-- Release notes generated using configuration in .github/release.yml at v0.3.6 -->

What's Changed
- (Preliminary) Add new function `calc_ept_corrected_e` to correct EPT electron measurements for ion contamination

**Full Changelog**: https://github.com/jgieseler/solo-epd-loader/compare/v0.3.5...v0.3.6

<!-- Release notes generated using configuration in .github/release.yml at v0.3.5 -->

What's Changed
- Fix a bug occurring with cdflib<1.0

**Full Changelog**: https://github.com/jgieseler/solo-epd-loader/compare/v0.3.4...v0.3.5

<!-- Release notes generated using configuration in .github/release.yml at v0.3.4 -->

What's Changed
- Make `resample_df` independent of seppy
- Add test functions & corresponding GitHub Actions
- Remove 3.6 & 3.7 from supported Python versions because they're not srupported by sunpy any more
- Clean requirements (hopefully fix 17 for conda-forge)

**Full Changelog**: https://github.com/jgieseler/solo-epd-loader/compare/v0.3.3...v0.3.4

<!-- Release notes generated using configuration in .github/release.yml at v0.3.3 -->

What's Changed
- Bugfix circular import caused by adding function `resample_df` from [seppy](https://github.com/serpentine-h2020/SEPpy)

**Full Changelog**: https://github.com/jgieseler/solo-epd-loader/compare/v0.3.2...v0.3.3

<!-- Release notes generated using configuration in .github/release.yml at v0.3.2 -->

What's Changed
- Add function `combine_channels` to combine fluxes of multiple adjacent energy channels of EPT or HET (fix 13)
- Obtain resampling function `resample_df` from [seppy](https://github.com/serpentine-h2020/SEPpy) (fix #18)

**Full Changelog**: https://github.com/jgieseler/solo-epd-loader/compare/v0.3.1...v0.3.2

<!-- Release notes generated using configuration in .github/release.yml at v0.3.1 -->

What's Changed
- Add omni-directional data (`viewing='omni'`) for loading EPT and HET data (https://github.com/jgieseler/solo-epd-loader/commit/1d26657757ab649369cf6746b8d955e6d2682816). This loads all four viewing directions and calculates an average of those following `('sun'+'asun'+'north'+'south')/4`
- Add tqdm progress bar to `calc_electron` (https://github.com/jgieseler/solo-epd-loader/commit/5599880d1a6cab2a9c1e987c1c82d9bc74adea10)
- Catch error when loading empty STEP data (https://github.com/jgieseler/solo-epd-loader/commit/ddc14cfacffc78ca1dde9287c406009cd0405fba)

**Full Changelog**: https://github.com/jgieseler/solo-epd-loader/compare/v0.3.0...v0.3.1