Introduced the so called reference position of each cell

and, of course, means to push cells to keep to their reference
positions. User may specify her desired reference position with
the config file. If none is specified, the position as it is
imported from the SOFA is used.


M    src/cell.h	- added support for the reference positions
M    src/cellSOFA.cpp - added implementation of the support

M    src/cell.cpp	- both initial cell mask obtaining functions reposition the cell

M    ISBI2015/config-smallVOI-lowSNR.ini
						- added sample specification of the reference position
						- some docs corrected (hopefully improved ;-)

M    src/scheduler.cpp  - patched error that forced scheduler to treat boundary
                          in the user supplied initial mask image as a cell