New functions

[coldfix]

Noting this down before I forget: From last discussion with @rdemaria , I think the following functions would be useful:

• Madx.call_in_process(func, *args, **kwargs), calls the given callable in the MAD-X process and transfers back the result. For a start pickle can be used to transfer over the stuff. Long-term something that can also deal with closures may be desirable.
• libmadx.select_by_type(sequence_name, elem_type) iterates (in C) through the sequence and returns list of element indices that match given type(s)
• libmadx.select_element_data(sequence_name, element_indices, field_name) retrieves (in C) the requested field(s) from all selected elements

Later on:

• provide more low level bindings to all MAD-X types that can be used for very fast access within the MAD-X subprocess