Add to main repository

[dzenanz]

I think this filter needs a bit more detailed documentation (it is slow for small intensity sigmas and large pixel types), more testing as described in #1, and is ready to be added to the main repository. Then this repository can be deleted.

[phcerdan]

I am happy to contribute it to ITK proper, but I agree a more careful review with higher quality standards has to be put in place. The current state is taken directly from the IJ submission with just style updates.

[phcerdan]

Merged in ITK: InsightSoftwareConsortium/ITK#4527