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.
Set version to 1.6.0
Set version to 1.6.1-dev
Add CubeList.merge_cube().
Includes/builds-on contributions from bblay, pp-mo, and cpelley.
MergeError docstring tidy
Review tweaks
Add tests for error_on_merge permutations.
Move _CoordSignature.match_payload to _CoordPayload.match_signature
Make DuplicateDataError subclass MergeError
Merge pull request #991 from rhattersley/mergefail

Add CubeList.merge_cube().
Support lazy operations in cube.collapsed.
test fix
Changed iris.cube.Cube.coord() and iris.cube.Cube.coords() to accept coord name
string or coord instance as first kwarg

small fixes
Merge pull request #932 from niallrobinson/coord_dims_with_str

Changed Cube.coords() and Cube.coord_dims() to accept a name or coord
Forced .travis.yml to latest biggus (with 'var' and 'Aggregation.masked_array').
Merge pull request #996 from pp-mo/lazy_collapses

Support lazy operations in cube.collapsed.
Merge pull request #993 from bblay/fix-coord-collapsed

Fix coord collapsed
Merge commit 'f47268f' into merge16fix

Conflicts:
	lib/iris/__init__.py
	setup.py
Merge pull request #997 from SciTools/v1.6.x

v1.6.x
concat mask