Class _Position2Tuple

Create callable object that converts fractional coordinates to
a tuple of integers with given precision.  For presision close to zero
it will return a tuples of double.

Note: Helper class intended for local use only.

Data members:

eps -- cutoff for equivalent coordinates.  When two coordiantes map to the
       same tuple, they are closer than eps.

Initialize _Position2Tuple

eps -- cutoff for equivalent coordinates

Convert array of fractional coordinates to a tuple.

xyz -- fractional coordinates

Return a tuple of 3 numbers.