Optimize adding TF

[kujaku11]

There is a slowdown in adding transfer functions, that mainly seems to do with updating the survey metadata each time a new transfer function is added. Add options to no update each time and just update at the end. This PR helps speed up things to resolve issue https://github.com/MTgeophysics/mtpy-v2/issues/44.

• [x] Speed up recursive search for station summary in mth5/groups/station
• [x] Overwrite write_metadata in mth5/groups/survey.SurveyGroup this needs to be done because the metadata property fills in the entire survey metadata including stations, runs, channels. However, the survey metadata is just for the survey, so need to override what metadata is being written.
• [x] In mth5/groups/transfer_function add update_metadata keyword to not update the survey metadata when a transfer object is added. Propogate that keyword to other functions.
• [x] Added kwargs to be passed to mth5/mth5.MTH5.open_mth5 so that the user can add other options for opening an h5 file.

[kujaku11]

• Need to update make synthetic tests to test for a full channel summary.
• Should add a test of adding transfer functions This is done in PR #243