TransferFunctionHeader should take Station, Channel objects

[kkappler]

Currently this is just using some id strings like "PKD" and "ex", "ey" to denote info about the processing channels.

However, information about the instrument locations and orientations must eventually be passed on to the output data structure. To do this we will need to carry along the mt_metadata objects.

[kkappler]

In the longer-term we want the full metadata objects stored in the header. In the shorter term for testing we can have been using string ids. A good way to manage the transition is to leave a placeholder for the metadata objects and the ids. The id methods can later be overwritten by properties that access the metadata.

For example, local_station can be objects of type mt_metadata.transfer_functions.tf.station.Station() or None
Then local_station_id can return self.local_station.id if it is available, but otherwise return
self._local_station_id.

[kkappler]

This has been solved by the TF class from mt_metadata. Also TransferFunctionHeader will be deprecated rather than improved (see issue #109 )