Make .madx with relative pathes usable

Currently, .madx files containing relative pathes don't work. The easiest solution I can think of is to change the path temporarily before CALLing a file. NOTE: this needs to be done in the remote session.

This may need to be changed in the PyMAD code.

I think the simplest solution is to add a `chcwd` method to `cpymad.madx.Madx`.