Add optional pfts argument to run.sensitivity.analysis() (fix/workaround for #3154)

[Aariq]

Description

Adds a pfts argument to run.sensitivity.analysis() to allow the user to specify a subset of PFTs in the settings object to run sensitivity analysis on.

Also prevents this function from dumping objects into the global environment (and possibly overwriting things without any warning) by specifying the environment for load()

Motivation and Context

Doing sensitivity analysis with a large number of PFTs generates a large number of runs and to save on compute time, I "manually" removed SA runs for PFTs I'm not interested in. run.sensitivity.analysis() interprets this as an there having been an error in model runs and doesn't generate all the files it is supposed to (in particular, no sensitivity.results.Rdata) (#3154).

Review Time Estimate

• Immediately
• Within one week
• When possible

Types of changes

• Bug fix (non-breaking change which fixes an issue)
• New feature (non-breaking change which adds functionality)
• Breaking change (fix or feature that would cause existing functionality to change)

Checklist:

• My change requires a change to the documentation.
• My name is in the list of CITATION.cff
• I have updated the CHANGELOG.md.
• I have updated the documentation accordingly.
• I have read the CONTRIBUTING document.
• I have added tests to cover my changes.
• All new and existing tests passed.