Solo-epd-loader

- Fix some `cdflib` usage that could cause a bug when run in Windows

- Replace usage of `cdf2df()` from [HelioPy](https://github.com/heliopython/heliopy) with `read_cdf()` from [SunPy](https://github.com/sunpy/sunpy) for loading EPT and HET data files
- Obtain FILLVAL for STEP data dynamically (instead of hard-coded)
- More detailed description of caveats in the readme
- Bugfixes in example code in the readme

- `autodownload` option is now able to create the necessary subfolders in the `path` directory
- Parameters `startdate` and `enddate` are now allowed to be `datetime` objects

- Remove dependency on heliopy because of its end of development