Quantics grid rounding problem

[thkloss]

grid::Quantics may return the wrong bitvector for a given coordinate.
In the given test example, we sample over all 2**nBit gridpoints and compare the corresponding bitvector from the grid.coord_to_id(x)
method and a reference implementation. Both differ from time to time, and one can check by looking at the actual output that the grid.coord_to_id(x) method does not always increment the bits properly.
This behaviour is probably due to some rounding issues, due to the intermediate conversion to a floating point number.

[thkloss]

run the test with: xfac_test "Test quantics rounding"