Use setup_requires for numpy

Enable to invoke `setup.py` without prior installation of numpy. This makes the requirements.txt file unnecessary which was used in a somewhat confusing manner. In the future the file can be used to to define stable version configurations instead.

Also, I have first simplified the MAD-X path discovery and then removed it altogether. If MAD-X is installed system-wide, it should be usable without manually extending the path variables anyway. If it resides in any nonstandard location, I think it's not too hard to use the --madxdir. This greatly simplifies the code in setup.py and I like simple. If you prefer the additional convenience of the old discovery mechanism, we can strip dddf1d4 from this PR.