FIX: convert to float to avoid integer division
FIX: convert to float to avoid integer division

This bug appears when the frametimes are integer values: in this case
integer division cancells the result and the drifts become
constant. This fixes the problem.

alexis-roche/nipy,bthirion/nipy,arokem/nipy,alexis-roche/nireg,alexis-roche/niseg,bthirion/nipy,arokem/nipy,alexis-roche/nireg,alexis-roche/nipy,bthirion/nipy,alexis-roche/nipy,alexis-roche/register,arokem/nipy,bthirion/nipy,nipy/nipy-labs,alexis-roche/nipy,alexis-roche/register,alexis-roche/niseg,nipy/nireg,nipy/nireg,nipy/nipy-labs,arokem/nipy,alexis-roche/register