Change from Exception to warning when the rt's ReferencedSOPInstanceUID is different from the CT's SOPInstanceUID

[Zhack47]

Problem:

Sometimes the CT used by the radiologist for the manual segmentation of ROIs might not be the one put in the final database.
To give a concrete example, one might have an injection CT and one without injection, do the rtstruct on the injected one and put the non-injected one in the database.

Consequence

In this case the CT's SOPInstanceUID is different from thee RT's ReferencedSOPInstanceUID, even though the space is the same.

At lines 78-82, the check made raises an error if these are not identical.

Proposition

It would be great if the user could chose whether this should be a warning or an Exception, through a parameter passed to the RTStructBuilder (like strict_check_SOPInstanceUID=False)

If you think it's a good idea, I will submit a pull request for it.

[asim-shrestha]

Closed by #64

Thanks again for working on this!