Implementing IfcOffsetCurve and all its subtypes

[SamuilsRulovs]

Fixes #557

• [x] Refactor IfcOffsetCurve
• [x] Add Doxy-comments
• [x] Implement IfcOffsetCurve2D
• [x] Implement IfcOffsetCurve3D
• [x] Implement IfcOffsetCurveByDistances
• [ ] Create unit tests

[SamuilsRulovs]

After the latest changes, OIP did not accept implementations of the IfcOffsetCurve3D and IfcOffsetCurveByDistances. I managed to update IfcOffsetCurve3D, but IfcOffsetCurveByDistances is not yet changed. Therefore IfcOffsetCurveByDistances is wholly commented out for now.

[SamuilsRulovs]

I tried to implement IfcOffsetCurveByDistances, however, I have some questions regarding the attribute OffsetLongitudinal. I would be very glad if we could discuss this attribute. Also, there is a question regarding example files, so far I couldn't find any for these entity types

[SamuilsRulovs]

I haven't implemented the whole solution, yet. However, I hope, that I am going in the right direction

[SamuilsRulovs]

Please review my changes to the convertIfcOffsetCurveByDistances function

[SamuilsRulovs]

As I was trying to include example files I stumbled across new issues regarding the implementation of the OffsetCurve. Some parts of the code come into conflict with already implemented parts. I am sure, that the biggest part of this problem is due to the use of different versions of OIP. (Part of the CurveConverter is only available for the IFC4x1 version of OIP, however, it is not applied for some parts of the code, thus OIP is trying to work with it even in newer versions. ) This causes problems with OffsetCurve. Until this issue is not solved, Offset Curve cannot work correctly. I couldn't solve it, so I decided to comment the OffsetCurve out.

[jschlenger]

Putting most of the code in comments allows to safely merge this pull request. I'm sure it is a good starting point for further implementation.