Implementation variable beam center (and small modification of SampleChangerMockup)

Hi Guys,

this is the implementation of the variable beam center. It changes HutchMenuBrick and the MiniDiff hardware object. Basically MiniDiff is extended by two small methods delivering the beam center instead of hardcoding the image center in several places.
For the MD2, the beam center is then read using two attributes from the device server. NOTE: The extension is not implemented for the Java MD2 application because I don't know the proper commands.

The changes also affect the 3-click and auto-centring procedures, so testing is needed at the ESRF too!

In some cases, the beam position is not updated correctly after a zoom level change using the MD2 with the v4 application, maybe because the new position is calculated too late inside the MD2 application. As a quick resolution, I have introduced a small delay before drawing the beam which at least seems to improve the situation. Of course this has to be resolved asap.

Best wishes,
Michael