BUG FIX: Serialize collapsed coordinates by datatype, not units

This fixes a bug introduced by #847, which is encountered when attempting to
collapse along a dimension coordinate with unknown units. In such a case, the
coordinate value is converted into a string, which fails for dimension
coordinates because dimension coordinates must be numeric.

In contrast, this new patch chooses how to serialize a collapsed coordinate by
its datatype instead of its units. String point values are joined by '|',
whereas all other point values (which are presumed to be numeric) are replaced
by the mean.
test fix
Merge pull request #993 from bblay/fix-coord-collapsed

Fix coord collapsed
Merge pull request #997 from SciTools/v1.6.x

v1.6.x
concat mask
lic date
nan fill
data_is_merged
np fill
Remove obsolete IrisTest._assert_cml().
Merge pull request #1006 from rhattersley/remove-obsolete-test-method

Remove obsolete IrisTest._assert_cml().
Show full reference file path on test failure.
Sphinx data documenter for instances.
Merge pull request #1009 from bjlittle/sphinx_opaque_agg

Sphinx data documenter for instances.
Merge pull request #1007 from rhattersley/show-path-in-test-failure

Show full reference file path on test failure.
Improved error message on unit conversion failure
Proof of concept for bounded forecast_period.
Add tests, fix bugs in proof-of-concept, and refactor.
Add integration test for format interoperability.
Update test results