Unsafe concatenate

[ScottWales]

Working with lots of files is a pain - xarray opens each and every one, checking that file 1's latitude is the same as file 5068342's latitude.

Add a function that will just assume the data is sane - but if it's not, bad things will happen!

[codecov-io]

Codecov Report

Merging #8 into master will increase coverage by 1.06%.
The diff coverage is 90.47%.

@@            Coverage Diff             @@
##           master       #8      +/-   ##
==========================================
+ Coverage   64.74%   65.81%   +1.06%     
==========================================
  Files           6        7       +1     
  Lines         485      506      +21     
==========================================
+ Hits          314      333      +19     
- Misses        171      173       +2

Impacted Files	Coverage Δ
src/coecms/bigdata.py	90.47% <90.47%> (ø)

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