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.