Related to MMDB-1544 on Medici.

The reason for the proposed change is that currently, there are two extractors in versus-image that seem to be accepting RGBPixels, but really want a specific type of image. As a result, if a user chooses one of the extractors and passes a wrong type of image, an exception is thrown.

versus-image/RGBHistogramExtractor
expects pixels to have length =3. Throws exception otherwise. (In other words, expects a color image and throws exception in case of gray-scale image).
versus-image/GrayscaleHistogramExtractor
Expects pixels to have length =1, throws exception otherwise (In other words, expects a gray-scale image and thrown exception in case of color image)

Proposed change:
On Versus side -
Should have an additional method for each extractor - call it Information or Explanation (but it's NOT the descriptor/feature, so one should be careful in choosing method name to avoid confusion). This information should be passed to Medici along with other info about extractors.

On Medici side -
The Admin page on Medici allows user to create an index by selecting adapter, extractor, and measure from drop-down lists. This page should have a pop-up window appear as user hovers a mouse above an extractor name. The pop-up window should have some information about an extractor, e.g. :"This extractor accepts only gray-scale images" OR "Color images only"

Displaying this information on Medici side when user selects an extractor would help choose correct extractor for given image.