Add Fortran unit tests

Developers at Respec wrote up some unit tests to test the Fortran code. The unit tests are written in Fortran and we want to bring them over into this package. The most straightforward way to do this is to rewrite their units test using R and the testthat library. There are two test suites with 6 unit tests in each suite.

• Suite 1: https://github.com/respec/FORTRAN/blob/master/f90apps/peakfq.new/suite1.F90
• Suite 2: https://github.com/respec/FORTRAN/blob/master/f90apps/peakfq.new/suite2.F90

Some of the unit tests read in CSV files so we will most likely have to host these locally so the tests can call them:

• example_truth.xlsx
• example_intervals.xlsx

Additional Notes:

• Notes from call with Respec: https://doimspp.sharepoint.com/sites/GS-UMidWIM/_layouts/15/doc.aspx?sourcedoc=%7B8e6d9b07-f39e-4fc4-9301-4bf5874ee7b9%7D&action=edit
• Notes on unit tests: https://doimspp.sharepoint.com/sites/GS-UMidWIM/_layouts/15/doc.aspx?sourcedoc=%7Ba482b192-aba8-4d1e-81b9-5fc5af554fd3%7D&action=edit