Add getRGBVector3i() to every point type with RGB

Currently only some of the point types with RGB have methods to access
their RGB data as an Eigen vector.

This commit moves definitions of `getRGBVector3i()` and
`getRGBVector4i()` inside `PCL_ADD_RGB` macro so that every point type
with RGB field gets these functions.

Additionally, `getRGBVector4i()` is renamed into `getRGBAVector4i()` to
make it clear that it produces a vector where last value is alpha
channel.