Support use of Google Film for temporal interpolation

[gavinevans]

Addresses https://github.com/metoppv/mo-blue-team/issues/763
improver_test_data PR: metoppv/improver_test_data#118

Description
This PR adds two pieces of functionality:

• Extends the existing temporal interpolation to support specifying "google_film" as an option.
• Adds a ForecastPeriodGapFiller plugin and CLI for filling gaps in forecast period, or smoothing across transitions between different forecast sources. This is a slightly different set-up to the existing temporal interpolation plugin and CLI, which is expecting a start and end cube only.

File breakdown:

• envs/environment_a.yml - Add Tensorflow and Tensorflow_hub to environment to be able to use Google Film.
• envs/latest.yml - Add Tensorflow and Tensorflow_hub to environment to be able to use Google Film.
• improver/cli/forecast_period_gap_filler.py - A CLI for the ForecastPeriodGapFiller plugin. This is a sister CLI to the temporal-interpolate CLI but is intended to take a range collection of forecast periods and fill gaps, or smooth across transitions between forecast sources.
• improver/cli/temporal_interpolate.py - Modifications to add "google_film" as an option.
• improver/utilities/temporal_interpolation.py - Modifications to the existing TemporalInterpolation plugin and addition of the ForecastPeriodGapFiller and GoogleFilmInterpolation plugin. The GoogleFilmInterpolation plugin is accessed through either the TemporalInterpolation or ForecastPeriodGapFiller plugins.
• improver_tests/acceptance/SHA256SUMS - Checksum updates.
• improver_tests/acceptance/test_forecast_period_gap_filler.py - Acceptance tests for new CLI. This uses linear interpolation. The Google Film interpolation is more time-consuming.
• improver_tests/acceptance/test_temporal_interpolate.py - Add an example acceptance test that uses Google Film. This takes a bit longer than most other acceptance tests and relies upon accessing the Google FILM url.
• improver_tests/utilities/test_ForecastPeriodGapFiller.py - Unit tests for ForecastPeriodGapFiller plugin.
• improver_tests/utilities/test_GoogleFilmInterpolation.py - Unit tests for GoogleFilmInterpolation plugin.
• improver_tests/utilities/test_TemporalInterpolation.py - Unit tests for TemporalInterpolation plugin.

Testing:

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