Unstructured scheme iris source

[bjlittle]

This PR provides iris source GitHub artifact support, to allow testing against a specific iris GitHub repo branch name, tag name or commit sha.

It also contains conditional task execution, better caching, and controls for conda environment verbosity.

The flake8 and black linting tasks have been combined, and the long/variable running osx task as been retired.

[codecov]

Codecov Report

Merging #33 (3d338e8) into unstructured_scheme (bddb48e) will not change coverage.
The diff coverage is n/a.

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[bjlittle]

Left "as is" the esmf-regrid repo stated requirements are honoured, with no iris source customisation.

The way in which the iris source is controlled is achievable via one of 3 ways.

1. Setting the IRIS_SOURCE environment variable at the CLI or within a repo commit to .cirrus.yml or noxfile.py.
   The expected format of IRIS_SOURCE is github:<artifact>, where <artifact> can be any valid SciTools/iris repo reference, such as a branch name, tag name, or commit sha. Setting the IRIS_SOURCE in an esmf-regrid commit binds this configuration to that branch until it is backed out of change, but it may be overridden in two different ways, as shown later. The following examples highlight the different IRIS_SOURCE formats,

github:mesh-data-model  # the HEAD commit of the mesh-data-model feature branch
github:master           # the HEAD commit of the master branch
github:v3.0.1           # the v3.0.1 tag
github:c34662c2a252c154f45f361d34ae8f1b4397b326  # a specific commit sha

2. Using the cirrus-ci.com configuration option for the esmf-regrid project via the cirrus-ci API through the browser. This allows you to create and override existing cirrus-ci environment variables dynamically without making a repo commit - and it is stateful. If used, this can be done on the developer owned cirrus-ci project, and/or the SciTools-incubator/iris-esmf-regrid project. The fuller details of how to perform this are provided separately below.
3. Providing the IRIS_SOURCE equivalent via the nox CLI. This option will override all above existing IRIS_SOURCE configuration options e.g.,

nox -s tests-3.8 -- --iris=github:<artifact>

[bjlittle]

The intent is to extend the format of IRIS_SOURCE to support similar fine grained control of the iris conda-forge package with the <channel>:<spec> format e.g., conda-forge:<3 or conda-forge/label/rc_iris:3.0.0rc0, or even simply specifying the channel but no spec e.g., conda-forge, and let conda resolve the package version.

[bjlittle]

The CODECOV_TOKEN is now configured via the cirrus-ci SciTools-incubator/iris-esmf-regrid project configurations accessible from the browser

[trexfeathers]

Tres cool 💐

[bjlittle]

[bjlittle]

@stephenworsley Just to confirm this PR "as is" makes no change in behaviour for iris-esmf-regrid testing i.e., it will always install iris from conda-forge as specified in the iris-esmf-regrid requirements file e.g., requirements/py38.yml.

To start using the capability of this PR in your unstructured_scheme feature branch, I'd recommend making the following change to the .cirrus.yml file:

This will result in your unstructured_scheme feature branch always using the mesh-data-model feature branch from iris. Note that, once this is cached, updates to the HEAD of mesh-data-model won't be automatically pulled in. You'll have to manually invalidate the nox_cache by bumping the NOX_CACHE_BUILD environment variable in the .cirrus.yml file.

HTH

[stephenworsley]

Looking good, just a couple odd documentation adjustments and we should be good to merge.