updated met2CF.csv;

added test and test input file. 

* function now handles datetime fields and uses our convention of 'days since 1700-01-01'
* does not require a CO2 field (now requires airT)
* added test file and wrote a simple test
* removed 'require' statements to load libraries that are already in package dependencies
* removed internal debugging statements

Note: this function does not use the `PEcAn.utils::mstmipvar` to generate standard variables. But probably should for consistency (and this will make it easier to update all variable naming conventions at once using the mstmip_vars dataframe).