make S3 methods out of the plotting functions so we can use FRASER in parallel

[c-mertes]

Currently the plotting function will be overridden by FRASER on load or vice versa because they are plain functions.

This will make it unfeasible to work with both packages at the same time.

The solution is to make S3 methods out of any methods shared with FRASER. Then we can use the packages in parallel. This would require a dependency to the registering package.

I guess OUTRIDER implements the S3 methods and FRASER depends on OUTRIDER.

Any thoughs: @vyepez88 @brechtmann ?

[c-mertes]

fixed in 0bff757