Add MeasurementWithSpheres extension

[cagatayalptekin]

[lassoan]

Thank you for your contribution. This is a simple but useful example of custom measurements.

To publish this extension, you would need to update the extension description in the top-level CMakeLists.txt file:

https://github.com/cagatayalptekin/SlicerMeasurementWithSpheres/blob/a867ce7e96984a11fdcedec0eed5704ddd16a7a8/CMakeLists.txt#L7-L13