Fix precision problem in BraggScattererInCrystalStructure "Position"-property

[MichaelWedel]

In ticket http://trac.mantidproject.org/mantid/ticket/11465, precision problems in calculation of equivalent positions in space groups were addressed.

The class Geometry::BraggScattererInCrystalStructure handles atom positions by exposing a PropertyWithValue``.

During tests with the structure of SiO2 (trigonal, space group 154) and PoldiCreatePeaksFromCell, it turned out that for correctly handling coordinate values of "2/3", this mechanism is not precise enough. No matter how accurate the position is given in the string passed to the algorithm, it is at some point cut off at 6 digits and leads to incorrect equivalent positions.

The property should be converted to a string that is interpreted using the multi-return value feature of muParser. This has the advantage that users can specify coordinates like "1/4", "1/3", "1/2", "2/3" etc. directly and they are evaluated by muParser down to double precision, preventing problems as described above.

On the downside, it will not be possible any more to assign V3D-object directly to the property using setProperty, so the unit tests using this property have to be adjusted.

[MichaelWedel]

This issue was originally trac ticket 11510