This PR targets: #253, #76 and to some extent, maybe #208
It features:
* api: readPixel on a HiPS object method taking x, y pixel values on the
screen. When no screen coo are given, the one of the center screen is
used.
* api: HiPS.probe a general method using HiPS.readPixel under the hood.
It allow to probe the survey pixel values on a pixel, along a screen line and
along a great circle arc on the sky
* fix: readPixel could throw an exception if the tile has not been
received. If so, now, we return a JS null value
* fix: retrieve pixels on fits HiPS bitpix=-32
* feat: a new aladin mode called TOOL_COLOR_PICKER.
* tool: showColorPickerControl: when clicking on it, enter the
TOOL_COLOR_PICKER mode. The user can move the mouse on a pixel to know
its value. With a click, the pixel value is copied to its clipboard
* fix: restore samp button to connect to a hub
* fix: call redraw when calling gotoRaDec to update instantly the
imageCanvas #208
* a new global readCanvas method on the Aladin object that
will simply read the final image Canvas. User can give a pixel coo, a
line (2 pixel coos), a rect (2 pixel coos for the box)
