Point verification workflow documentation

[andrew-MET]

The workflow for point verification in harp is reasonable straightforward. However this needs to be properly documented in a vignette - it is important that users understand how to use non standard evaluation for column names in a programming context using the curly-curly {{}} notation. Also, when the use the wrapper function for memory intensive operations needs to be clarified.

Furthermore, options such as grouping scores, taking observation errors into account, bootstrapping and other such things need to be taken into account,