str representation of numpy scalars in cube summary

[ukmo-ccbunney]

🚀 Pull Request

Fixes: #6692

Updates cube_summary.py to use str representation for numpy scalar attribute values in Cube and Coord summaries.

For a cube attribute like this:

cube.attributes['numpy_scalar'] = np.float32(123.456)

The cube summary will look like this:

<snip>
Attributes:
        scalar                      123.456

rather than this:

<snip>
Attributes:
        scalar                      np.float32(123.456)

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 90.11%. Comparing base (955e247) to head (cc870f1).
⚠️ Report is 51 commits behind head on main.

Additional details and impacted files

@@           Coverage Diff           @@
##             main    #6966   +/-   ##
=======================================
  Coverage   90.11%   90.11%           
=======================================
  Files          91       91           
  Lines       24912    24912           
  Branches     4675     4675           
=======================================
  Hits        22449    22449           
  Misses       1684     1684           
  Partials      779      779           

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[ESadek-MO]

LGTM thanks @ukmo-ccbunney