[common] Make color tables of LUT_GLASBEY LUT_VIRIDIS constexpr public

Currently there exists ColorLUT to wrap both tables. As the API is really similar to to std::array we could remove the class and just publish the values as array somehow

Some steps which are necessary for it:

• Use std::array instead of C arrays #4489
• Make pcl::RGB constexpr (necessary for next step)
• Change array from unsigned char to pcl::RGB, so the method ColorLUT<T>::at does not anymore need to add the dummy 4. value (as pcl::RGB is using 4 bytes and not 3)
• Move arrays to header(s), so they can be really constexpr

Start by reading the existing ColorLUT implementation and the definition of pcl::RGB, then review the completed std::array change in #4489. Trace how LUT_GLASBEY and LUT_VIRIDIS are declared and accessed. Done means pcl::RGB is constexpr, the tables use pcl::RGB, and the arrays are public constexpr in headers without the dummy fourth value.