Fixes in OEP/CPF Downloaders

[stefanlinden]

Hi,
First of all, thanks for the package, I have found the propagator to be very useful.
However, as also stated in issue #5, the downloaders are potentially powerful but broken at the moment. This PR fixes these issues to get in line with changes due to mainly CCDIS switching to CPF v2.
The changes are as follows:

1. The dependency requests was missing and has been added. It has been added with a version wildcard though, but it should be fine with at least all versions from the last couple of years.
2. The EOP data provided by the IERS is not available through FTP anymore. Fortunately, they still provide it as an HTTPS download, so I have updated the code. Note however that on my system, astropy seems to download the EOP automatically anyway when it is not set, so the downloader may be superfluous?
3. For the downloads from both CCDIS and EDC, it will try to download CPFv1 files if the specified date (if any) is from before the 1st October 2021 and CPFv2 files if it is after that date. This is because these organisations formally adopted v2 on this with various differences. Most notably, the 'current' datafiles are only available in CPFv2.
4. The web scraper for CCDIS has been modified to remove the checksum files from the search tree in a more direct fashion, as it varies a lot whether these files are available or not. This makes the downloads from CCDIS more robust.
5. Various small fixes in the code and its formatting. Most files are now fully pep8 compliant, but I haven't looked at all files (yet).