Add madx.chdir to change current working directory
Change directory when doing CALLs by default

I think this will more likely match any user expectations when CALLing
files with CALL instructions using relative pathes inside them.
Remove obsolete docstring portion

This is obsolete due to 38908cc
Enable RPC of functions in arbitrary modules

This was originally thought as a generalization to make 'os' importable
in the remote process in order to perform ChangeDirectory operations.

Now, I decided to add 'chdir' and 'getcwd' to 'cpymad.libmadx' for
convenience, so this commit is not a necessity for the moment. But I
still believe, that it can prove useful for third party tools that need
to perform special setup (e.g. in MadGUI).