now using discrete gaussian kernels

This is an important and subtle change which will effect nearly all of the detectors, especially when attempting to detect objects which are thin or small (only 1-6 voxels wide). Hopefully this is an improvement.

Unfortunately the new code now requires a c++17 compatible C++ compiler, such as gcc7 or clang5 or later.
(This is because I use some functions that were recently added to <cmath>. If this is a problem, let me know, because there are workarounds for older compilers, such as using the "boost" library.)

Unfortunately, this also means that if you used an earlier version of this program, you may have to adjust your thresholds to obtain the same results you had earlier (especially if you were detecting small or thin objects).

There were several other significant changes which are invisible to the user:

• I cleaned up the VISFD library C++ code (simplifying the API somewhat)
• I added move constructors for C++11 compatibility