Add function to change directory in the remote process

When executing MAD-X commands that depend on the filesystem, it is useful
to be able to control the directory that is used by the remote MAD-X
instance.

This becomes especially important when trying to `madx.call()` files
containing `CALL` statements with relative pathes. I believe, the user
expectation is that these should work, which is why I modified the
`madx.call()` method to temporarily change the directory by default.

See also, [this related issue](https://github.com/coldfix/madgui/issues/26)
in madgui.