CorrectionMass added to data model.
Added relation  between Rod and CorrectionMass. If Rod is deleted the CorrectionMass is also deleted, if CorrectionMass is deleted, the Rod CorrectionMass relation is set to null.
Edited CorrectionMass class and model. 
Length replaced by xEnd and yEnd coordinates.
Do not checkout to this commit.
Preparation for calculating correction masses.