FMS2_io: Unpack data

[uramirez8707]

Description

• After reading the data, fms2_io checks for the add_offset and scale_factor attributes and unpacks the data.
• Adds tests to make sure this capability works as expected.
• Work around in amip_interp for reproducibility

Fixes #1229

How Has This Been Tested?
CI
@J-Lentz tested this with LM4P

Checklist:

• My code follows the style guidelines of this project
• I have performed a self-review of my own code
• I have commented my code, particularly in hard-to-understand areas
• I have made corresponding changes to the documentation
• My changes generate no new warnings
• Any dependent changes have been merged and published in downstream modules
• New check tests, if applicable, are included
• make distcheck passes