PointCloudColorHandlerCustom ctor should accept colors in [0..1] range or have char arguments

Currently PointCloudColorHandlerCustom accepts three doubles in constructor to define a custom color. Internally they are then casted (as-is) to char and used for coloring, meaning that expected range is 0..255. This is confusing, counter intuitive, and worse of all, not documented.

We should do either of the following:

1. keep double arguments, but assume they are in [0..1] range
2. switch to char arguments

I prefer the first option since the interface will be in line with the rest of the visualization module (e.g. specification of point cloud color through setPointCloudRenderingProperties). However, option (1) breaks existing visualization code. This can be mitigated to some extent by issuing a warning if the values are outside expected range. Option (2) should be safe and only cause type compiler type warnings.

Start with the PointCloudColorHandlerCustom constructor and compare its interface with setPointCloudRenderingProperties in the visualization module. Determine whether the constructor should use [0..1] doubles or character arguments, including how existing callers are affected. Done means the selected behavior is implemented consistently and the accepted range or compatibility warnings are documented.