Simplify DIFFPY_VERSION formula to M00m00p.

DIFFPY_VERSION is an integer macro that encodes the (M)ajor, (m)inor
and (p)atchlevel as (1000000 * Major + 1000 * minor + patchlevel).
Tags must match  v[[:digit:]]  to define version number.

This adapts to vN.M[...] naming convention for version tags.