corr_fun function documentation and input requirements unclear

[moylettsinead]

The corr_fun function appears to have unclear documentation regarding its input requirements. The function expects a data frame, but the intuitive usage would be to pass two vectors for correlation analysis.

Function requires parameters that aren't mentioned in the initial error message
Parameter 'nx' and 'ny' are required but not documented clearly
Input requirements are different to other correlation functions
Error messages don't guide users to correct usage

Update function documentation to clearly list all required parameters:
df: input data frame
nx: name of first variable
ny: name of second variable
method: correlation method
Add examples in documentation showing correct usage
Improve error messages to be more informative

[moylettsinead]

openjournals/joss-reviews#7319