Fixes the zero/nan velocities for old plate reconstructions

It fixes the zero or nan velocities when the time we ask to calculate the velocity is an older limit of the age of the finite rotations given to plates (included in the rotational file).

As a default, we calculate the euler pole using two finite rotations at (time, time+1). With this fix, when the time we are want the velocities is the last listed in the finite rotations in the rotational for a given plate, we use the approximation of (time-1, time).

This is in line with what gplates offers. And obtains the same results as calculating the velocities using pygplates for conflicting times and platesID.