[FB] default_units: Update merge-base to commit 1873ce14

[abooton]

Update default units feature branch to base commit 1873ce1 (current head of master)

(Duplicate of #3792, but merging with --no-ff instead of --squash in order to preserve the commits)

[abooton]

The default_units feature branch merges into upstream/master without conflict when merged via the commandline:

git checkout upstream/master -b  default_units_base1873ce14
git merge --no-ff upstream/default_units

However, when trying to merge by creating a PR via GitHub, GitHub indicates a conflict in the file: docs/iris/example_code/Meteorology/lagged_ensemble.py
which no longer exists in upstream/master.

As such, the conflict can not be resolved simply because there aren't any conflicts!

Therefore, I have created a new feature branch, upstream/default_units2, branching at merge-base 1873ce14. I then merged in the original feature branch (as outlined above). I confirmed that the docs/iris/example_code/Meteorology/lagged_ensemble.py does not exist (as in master).

When merged in default_units2, this should replace default_units, which will then merge into upstream/master correctly.
This will preserve the original commit-ids that were actually reviewed (rather than rebasing that alters the commits) and GitHub will preserve the issue links and commit information.

[abooton]

This is identical to #3792. Comparing this default_units2 branch with master confirms that the 5 commits made to the feature branch have been preserved whilst updating the branch to head of upstream/master