Create SpecularDetector

Along with GISAS and off-specular instruments, specular instrument should have its own detector with dimensions, distance from sample and analyzer.

After that all analyzers will belong to detectors and IBeamScan::analyzerMatrix() can be removed (see also #651 (closed)).

In the future, the presence of the detector dimensions will make it possible to account the scattered intensity caught together with specular beam (#33)