Fix cf.aggregate failures when using the 'dimension' keyword

[davidhassell]

Fixes #283

[sadielbartholomew]

One comment (see in-line), otherwise all good:

This is simply a bug in code that was not covered by the units tests.

Bug fixed and a new unit test added which captures the bug (fails on the current master and not here).

[sadielbartholomew]

Note that the CI test failures are both solely due to the following error:

FileNotFoundError: [Errno 2] No such file or directory: '/home/runner/work/cf-python/cf-python/main/cf/test/test_file.nc'

so somehow the given test file is not being generated. I will look into that separately. Merging now.