Fix type piracy with Colors.RGB

[j-fu]

Colors.RGB gets methods for string and symbols in GridVisualizeTools.
This is type piracy. May be can be replace by parse() (see Colors.jl docs).

[jpthiele]

I think the only real occurence in this repo might be

GridVisualize.jl/src/meshcat.jl

Line 166 in 832bac8

rgb(v) = RGB(v, 0.0, 1.0 - v)

Otherwise rgbtuple is used which is defined in GridVisualizeTools.jl/src/colors.jl
so I think we should rewrite that function to use parse, right?

[jpthiele]

See: WIAS-PDELib/GridVisualizeTools.jl#12
and please check if everything still works as expected.