Implementation of map_indices_flag & argument to copy metadata to dictionaries

[MaxvandenBoom]

Hi!

This PR has two enhancements for the read_mef_session_metadata, read_mef_channel_metadata and read_mef_segment_metadata functions:

1. Finishes the implementation of the map_indices_flag. This optional argument was already present but did not do anything, now it does.

2. It adds an additional optional argument (copy_metadata_to_dict) to copy the metadata solely into standard python dictionaries and return that to the user; instead of returning a dictionary that often branches into numpy structured array objects that have pointers to underlying C data.

• With the copy_metadata_to_dict option set to true, the data is copied into python objects that own their own data while the complete session/channel/segment is cleared in c. This has the advantage that no separate call to C is required to empty the struct that underlies the returned object. The returned python object can just be deleted(or None'ed) which frees all the data.

• To ensure backward compatibility, the copy_metadata_to_dict options is False by default, mapping to numpy structured array objects using exactly the same routines that were always there.

• The dictionaries have the exact same keys as the numpy structured arrays. So for example: session_md['session_specific_metadata']['number_of_video_channels'] will work for both ndarrays (copy_metadata_to_dict=False) as with dictionaries (copy_metadata_to_dict=True)

• An addition advantage of copy_metadata_to_dict is that it makes inspection of the structure slightly easier: ndarray: dict:

Hope these enhancements are acceptable @cimbi I'll do one last PR after this one to see if I can fix Tom his issue, thanks!!

Best, Max

[MaxvandenBoom]

@cimbi Guess there's still some issues to resolve to pass the tests, I'll let you know when it's ready

[cimbi]

The tests are passing. Merging...

[tjr1]

This is nice upgrade Max! I'm going to use this!