New subtool: Fre pp histval

[Ciheim]

Describe your changes

Issue ticket number and link (if applicable)

Checklist before requesting a review

• I ran my code
• I tried to make my code readable
• I tried to comment my code
• I wrote a new test, if applicable
• I wrote new instructions/documentation, if applicable
• I ran pytest and inspected it's output
• I ran pylint and attempted to implement some of it's feedback
• No print statements; all user-facing info uses logging module

[ceblanton]

Some kind of test needed. Minimal test, with tiny cdf Netcdf example.