Monkey patch iris and speed up crunching

[znicholls]

Pull request

Please confirm that this pull request has done the following:

• Tests added
• Documentation added (where applicable)
• Example added (either to an existing notebook or as a new notebook, where applicable)
• Description in CHANGELOG.rst added

Adding to CHANGELOG.rst

Please add a single line in the changelog notes similar to one of the following:

- (`#XX <http:https://link-to-pr.com>`_) Added feature which does something
- (`#XX <http:https://link-to-pr.com>`_) Fixed bug identified in (`#XX <http:https://link-to-issue.com>`_)

Supercedes #68

[codecov]

Codecov Report

Merging #72 into master will increase coverage by 0.65%.
The diff coverage is 96.62%.

@@            Coverage Diff             @@
##           master      #72      +/-   ##
==========================================
+ Coverage   97.63%   98.29%   +0.65%     
==========================================
  Files          10       10              
  Lines        1184     1287     +103     
  Branches      165      184      +19     
==========================================
+ Hits         1156     1265     +109     
+ Misses         18       16       -2     
+ Partials       10        6       -4

Impacted Files	Coverage Δ
src/netcdf_scm/masks/__init__.py	98.94% <100%> (+3.15%)	⬆️
src/netcdf_scm/utils.py	100% <100%> (+2.89%)	⬆️
src/netcdf_scm/iris_cube_wrappers.py	99.02% <100%> (-0.14%)	⬇️
src/netcdf_scm/cli.py	95.61% <95.12%> (+0.97%)	⬆️
src/netcdf_scm/wranglers.py	100% <0%> (+4.16%)	⬆️

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[znicholls]

Twice the monkey @rgieseke :)

[znicholls]

@rgieseke I found the issues in iris which illustrate this issue: SciTools/iris#3333 SciTools/iris#3357