Change initialization of package and some renaming of key fns/methods

[jkingslake]

This is an attempt at a new and more sensible structure for the code.

These changes will mean that existing code using the package will most likely break. But the chnages required to fix that code will be very minor. Essentially just changing anything like

import ApRESDefs
xa = ApRESDefs.xapres(loglevel='debug')

to something like

import xapres_package as xa
fd = xa.load.from_dats(loglevel='debug')

The main changes are

• ApRESDefs is renamed to load
• functions that are not related to loading data are moved from load to another module, utils. These include phase2range, sonify, dB, coherence, and generate_range_diff
• .xapres has been renamed from_dats (i.e. it is an object containing data loaded from dat files)
• added a wrapper around from_dats so we have the option of just calling a function and not needing to get in to the object oriented approach: generate_xarray
• a large update to the main notebook demonstrating how to use the code usingXApRES.ipynb so that it includes this wrapper function, it loads the package in the new way, and it demos how to load data collected in attended mode and polarmetric data.

@glugeorge please let me know what you think, when you get a chance (no hurry :smile:)

[jkingslake]

The tests failed because I forgot to change the name of a method in the test scripts.