WIP: support for BTI/4D Magnes3600WH data

This includes the functions and the class from #248 
and binary readers -- disclaimer -- inspired by / refactored from python-megdata, a module hosted by the ynic york group.

Works:
- reading raw bti data and basic channel info from the so called PDF file
- reading headshape files

Written, but untested:
- reading user blocks, coil definitions and transforms from the so called config file

To be done:
- refactor the conversion utilities from #248 to deal with the new input sources
- add nose tests
- add sample data
- settle API questions from #383