Wrong LoG scale normalization in `blob_LoG`

[gschivre]

The correct scale normalization for LoG should be -σ^2 instead of -σ in the multiLoG function called by blob_LoG.

In addition, the docstring of BlobLoG says:

radius is equal to σ√2

but

radius is equal to σ√N for a N-dimensional image

is more accurate.

This error is easy to fix and I can handle it on my own (please note that this will be my first PR so any guidance is much appreciated!).