Prevent std::normal_distribution being called with sigma=0.0

[robbietuk]

Changes in this pull request

Add a guard to DetectorCoordinateMap to prevent std::normal_distribution<double>(0.0, 0.0) being called.

Testing performed

Built STIR in debug on windows machine and build project using STIR in debug. Fixes assert 0.0 < sigma issue observed in #1485.

Related issues

Fixes #1485

Checklist before requesting a review

• I have performed a self-review of my code
• I have added docstrings/doxygen in line with the guidance in the developer guide
• [] I have implemented unit tests that cover any new or modified functionality (if applicable)
• The code builds and runs on my machine
• [] documentation/release_XXX.md has been updated with any functionality change (if applicable)

[KrisThielemans]

I cannot check from here if the actual calls check first if sigma is zero.

@markus-jehl can you please review?

[robbietuk]

The get_coordinate_for_det_pos() method does but this assert error/fix is in the constructor.

[markus-jehl]

This looks good to me! Makes no sense creating the distributions and generators if they are not needed.