A ggplot with a geom_tile layer providing a plot of the
bivariate functional data object as a heat map.
Arguments
bifd_obj
A bivariate functional data object of class bifd,
containing one single replication.
type_plot
a character value
If "raster", it plots the bivariate functional data object
as a raster image.
If "contour", it produces a contour plot.
If "perspective", it produces a perspective plot.
Default value is "raster".
phi
If type_plot=="perspective", it is the phi argument
of the function plot3D::persp3D.
theta
If type_plot=="perspective", it is the theta argument
of the function plot3D::persp3D.