The free variants of operator/ and operator* now yield new
type of point<T> with T promoted to type common for both operands.
Add missing member operator*=, for symmetry.
Add template function iround(point<T>) as no-op sink for point
specialisations for T of integer types.
Add explicit casts of arithmetic intermediate results to avoid
compilation warnings.
Replace redundant point<T> construction in return statement with uniform
initialization.
Add static_assert validating range of axis_value integer template parameter.
Update test/point/point.cpp with basic checks of all point<T> members.
Fixes#153
