Adopting xarray as sole canonical format

[ashenoy463]

Reasons for:

• 1 time investment. no more dealing with text stream overhead, only optimised operations.

• Respect the poly-indexability of our data. We can index with timestep, box-time or atom_id, we could even form and track groups

• Naturally supports binning, chunking, averaging and mapping array functions

• Badaulat Dask; efficient parallel and out of core processing; optimised analysis functions can be developed with minimal effort. we do not need to reinvent the wheel.

[ashenoy463]

Can avoid #1 when filling in values.