change `default_aes(size=...)` to `default_aes(linewidth=...)`

[ammar-gla]

I don't know if anyone is maintaining this package, but due to a change in ggplot2 (tidyverse/ggplot2#3672 (comment)), the line-based geoms should reference linewidth instead of size, such as the below.

GeomGLALine <- ggplot2::ggproto("GeomGLALine", ggplot2::GeomLine, default_aes = ggplot2::aes( size= 3 * mm_to_pt)
to
GeomGLALine <- ggplot2::ggproto("GeomGLALine", ggplot2::GeomLine, default_aes = ggplot2::aes( linewidth = 3 * mm_to_pt)

Otherwise there is an issue where setting a static size value to a line outside the aes() field is completely ignored (i.e. when I do not want it to vary by group).