Pvweb protocols get lut image

This branch allows to retrieve a Base64 encoded image (either png or jpg) of the sampled lookup table for a given representation. Caller of the rpc method can specify number of pixels wide (number of samples) the image should contain, while the image will always have height of 1px. Caller of rpc method can also specify a custom range to use in sampling, else the method will use the current range of the lookup table associated with the representation.