Improve Coord and Cube attributes printout

[pp-mo]

During development of #4499, and discussion in #4501, it was suggested that when printing cube components with names/units/attributes (i.e. derived from CFVariableMixin) we should be listing any attributes in a one-per-line fashion, as Cube.__str__ already does.

This wasn't initially done in #4499 as I was concerned it introduced more change to an already complex PR.

On subsequent investigation though, it isn't really that complex, so this PR introduces that.
Results go from looking (schematically) like

DimCoord :  name / (units)
 . . .
    long_name: 'thing'
    dtype: float64
    attributes: {'string': 'this', 'array': [1.0 2.0 3.0], 'really_long_string': 'long long long long thing', 'float':3.2}
    circular: True

to

DimCoord :  thing / (units)
 . . .
    long_name: 'thing'
    dtype: float64
    attributes:
        string     'this'
        array      [1.0 2.0 3.0]
        long_name  'long long long long thing'
        float      3.2
    circular: True

Comparing the output of attributes in Cube listings to those in Coords, I also came to the conclusion that it would be better to always quote strings when they appear as attribute values -- that is, in Cube listings also.
Luckily that is very easy to do -- though there were quite a few tests to fix, so it's a bit noisy.

[pp-mo]

Added code to clip over-long strings, as in previous comments.

[lbdreyer]

Looks good! Thanks @pp-mo !