Implement "Luminous Flux" computation.

This implementation uses left-hand integration.

Closes #40.
Add "Luminosity" and "Luminous Flux" references.
Fix bug in doctest example.
Move "Luminous Flux" code to dedicated "colour.colorimetry.photometry" module.
Updated luminous flux computation to approximate integral using the trapezoid rule.
Updated luminous flux computation to discard values outside of the luminous efficiency function's domain.
Removed unnecessary unpacking.
Merge pull request #162 from colour-science/feature/luminous_flux

PR: Implement "Luminous Flux" computation.