Changed name of POLDI WorkspaceGroup

Having the same name for the grouped workspaces as for the raw data
resulted in segmentation faults.
Added code to represent POLDI's detector in an abstract way
Added license blocks to header files
Added centralElement to Detector
Added detector factory for POLDI
Started integrating detector code into autocorrelate
Fixed vector sizes in autocorrelation method

Since vector size will not change, the correct size is allocated
directly at construction.