add PLS scores plot for regression models

[grlloyd]

currently only PLSDA models are supported

[grlloyd]

this needs either
a) the PLSR object to return the scores as a dataset experiment object so that the chart for PLSD can be used or
b) a new chart object
I'd prefer to use a) but it requires some changes to PLSDA chart as well to make it more generic and less specific to PLSDA