Improve hexagonal shape bounding box

[Anders Markvardsen]

At the moment hexagonal shape bounding box is too large which causes

1. the initial view of Instrument to have the wrong focus
2. when rotating the instrument you don't see the wireframe properly

Fix this

[Anders Markvardsen]

(In [10390]) Fix: Improve hexagonal shape bounding box

In fact all what had to be done was adding a bounding box to the hexagonal shape in the IDF.

Added wiki documentation for bounding box, see http://www.mantidproject.org/HowToDefineGeometricShape#Bounding-Box

re #2694

[Anders Markvardsen]

James comment for testing updated HIFI: Yes, that IDF does display correctly - the instrument now fills the window when it is initially opened, and the wire frame boxes can be dragged round interactively. (though they do overlap in the middle as expected)

[Stuart Campbell]

This ticket has been transferred to github issue ​3541