PR: Implement "Luminous Flux" calculation

Refers to #40 

I went ahead and used left-hand numerical integration. This method allows for the use of non-equally spaced arbitrary lef functions and seems the most precise, short of trapezoidal integration. 

The test values are from the NIST CQS sheet, where I modified the luminous efficacy calculation by removing the denominator (reducing the calculation to the luminous flux) and multplying by the stepsize (dx=5nm). The resulting values aren't super precise compared to the NIST CQS xls sheet (only precise to 0.5 lumens), and it's not super clear why.