Fuzzy testing of index and value of regular axis

[HDembinski]

While we cannot avoid round-off error in the index and value methods of the regular axis, we want these two to be exact inverses of each other. In other words, this should succeed for all values of nbin, start, stop.

auto a = regular(nbin, start, stop);
for (int i =0; i < nbin; ++i)
   assert(a.index(a.value(i)) == i);

This needs to verified with fuzzy testing.