Make setup_orekit_curdir optionally load data from folder

This is a proposal to allow loading Orekit data from a folder. In my case, it speeds up the startup of my Python application a lot.

The two commits are actually 2 options:

1. The existing setup_orekit_curdir function was modified with an extra optional argument.
2. The os module is used to detect if the filename argument is a file or a folder.

merged

mentioned in commit b50d687d

Thanks!

We should maybe consider updating the download_orekit file also to extract it if selected.

Thank you for merging this.

As the new feature is more thought for users who maintain their own orekit-data folder, unzipping the archive from the download_orekit function is maybe not that important.

mentioned in issue #422