Multipanel plots overwrite original frame/grob classes

[stefaneng]

Multipanel plots have a class multipanel that overwrites the original classes. This prevents multipanel output from being used in standard grid functions. For example, without manually updated the classes a multipanel plot cannot be used in a multipanel plot.

https://github.com/uclahs-cds/public-R-BoutrosLab-plotting-general/blob/main/R/create.multipanelplot.R#L821C2-L821C30

This line should just add multipanel class to the class vector:

class(x) <- c('multipanel', class(x));

This would also simplify the print function as we can remove the class change:

https://github.com/uclahs-cds/public-R-BoutrosLab-plotting-general/blob/main/R/create.multipanelplot.R#L5-L13