Use a generic xarray-like Julia package for output

[sjdaines]

A better solution for storing model output would be to use a generic Python xarray-like Julia package that supports save/load to disk and analysis, plots etc, instead of the custom implementation (PALEOmodel.OutputWriters.OutputMemory) used at present (see #48, #47 for examples of the limitations of that approach).

This would allow a lot of code to be removed, and would improve interoperability with the rest of the Julia package ecosystem.

Currently it looks like the Julia ecosystem is still converging on what this xarray-like package (or packages) could be. Options include:

• DimensionalData.jl with extensions in Rasters.jl
• YAXArrays.jl