MOBT-719: Frequency to count for time period averages

[bayliffe]

This change adds a function and CLI to multiply average rates / frequencies within periods by the length of the period over which they are defined to yield an accumulation / count over the same period. This is to be used in the first instance for converting lightning flash frequencies into counts.

Acceptance test data: https://github.com/metoppv/improver_test_data/pull/49

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:

Project coverage is 98.43%. Comparing base (84a8944) to head (30da6f9). Report is 14 commits behind head on master.

Additional details and impacted files

```diff @@ Coverage Diff @@ ## master #2006 +/- ## ========================================== + Coverage 98.39% 98.43% +0.03% ========================================== Files 124 128 +4 Lines 12212 12489 +277 ========================================== + Hits 12016 12293 +277 Misses 196 196 ```

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