amend_attributes can now add to an existing value, and add timestamps.

[MoseleyS]

Addresses https://metoffice.atlassian.net/browse/EPPT-650

Updates amend_attributes method, used by standardise CLI, to allow attributes to be modified in these ways:

• "{}" in the new value will be populated with the existing attribute value on the cube.
• "{now:[date-format]}" will be populated with the current wall-clock, formatted as specified.

This allows us to update the history attribute of a cube and to append the current date-time and a string describing the process that has been applied.

Testing:

• [x] Ran tests and they passed OK
• [x] Added new tests for the new feature(s)

[codecov[bot]]

Codecov Report

All modified and coverable lines are covered by tests :white_check_mark:

Comparison is base (f77f534) 98.39% compared to head (5b5a7e4) 98.40%. Report is 1 commits behind head on master.

Additional details and impacted files

```diff @@ Coverage Diff @@ ## master #1973 +/- ## ======================================= Coverage 98.39% 98.40% ======================================= Files 124 124 Lines 12059 12067 +8 ======================================= + Hits 11866 11874 +8 Misses 193 193 ```

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