Clean the 'dyplot' S3 method

Currently the dyplot method is not well defined.

• replace the misnamed dyplot.default() function by an internal one (.dyplot())
• remove the dyplot.default alias from the 'dyplot' help page
• rename the dyplot.default.Rd file into dyplot.Rd
• rename the dyplot.default.R file into dyplot.R
• add the dyplot.PrepGR, dyplot.CalGR and dyplot.SimGR aliases and usages in the 'dyplot' help page
• bring the dyplot(), .dyplot(), dyplot.PrepGR(), dyplot.CalGR() and dyplot.SimGR() functions together in the same file