Initial growing season code

[davidorme]

Description

This PR is to work on issue #352, providing code to identify the growing season within a time series. The user should provide a logical array showing a daily time series of when conditions are suitable for growth, along with an array showing dates (align with solar/calendar stuff) and then this module provides tools to calculate the start, end and length of the growing season. The functionality should include a filter to remove short windows of unsuitable conditions.

• Implement filter_short_intervals to remove short intervals of unsuitable conditions. This should probably not filter unsuitable conditions at the start and end of the time series (or do so optionally).
• Implement growing_season(ts, dates, filter_unsuitable).

Fixes #352 (issue)

Type of change

• New feature (non-breaking change which adds functionality)
• Optimization (back-end change that speeds up the code)
• Bug fix (non-breaking change which fixes an issue)

Key checklist

• Make sure you've run the pre-commit checks: $ pre-commit run -a
• All tests pass: $ poetry run pytest

Further checks

• Code is commented, particularly in hard-to-understand areas
• Tests added that prove fix is effective or that feature works