Update rainforests tests to run only when required modules are present

[benowen-bom]

Addresses #2348.

A number of acceptance tests fail due to missing imports. This PR adds import skips to the specific tests that are failing to ensure the run only when the required module is present.

This also adds a change to the test_apply_rainforests_calibration that moves the import from module level to function level to ensure that the tests are listed in pytest run as skips.

Need to fix failing test on the conda-forge environment
The failing test is unrelated to the fix in this PR. I've opened another ticket on this issue; see #2351.

Testing:

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

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[jdjonathandavies]

Overview

This is my first review on the IMPROVER repo. I have:

• Read the code and didn't feel the need to post any in-line comments for suggested alterations
• Ensured that the unit tests ran
• Ensured that the acceptance tests also ran
• The Acceptance Criteria have been met
• The definition of DONE is satisfied
• The testing on the issue is clear

Details of checks

In order to check that the PR addresses the issue of tests being marked as skipped when libraries are missing I have:

• Checked out the branch
• Rebased it onto the latest master
• Updated my local checkout of improver_test_data
• Ran the tests (which passed)
• Edited the changed files to request libraries that don't exist
• Re-ran tests and confirmed that they all skip in that circumstance