Use P676 Eqn 11/12 and integrate.quad to simplify code and resolve w…

[alastairUK]

…hat to do when h1 > 0

Basically use the code in -12 Eqn 11/12 rather than the approximation.

As the footnote says:

"Equation (11) can be evaluated using various methods depending on the implementation: e.g. a) the integral
function in Matlab, b) the quad function in Octave, c) the quad function in Python, d) several Numerical
Recipes functions, and other equivalent methods"

This has multiple advantages

1. It's no longer an approximation
2. It resolves what to do when h1 > 0 (which is not clear to my eyes in the other method although I can guess what it should do)
3. It fixes a bug in the current version where the value being used is NOT what the Rec says:

If you use this new method and compare to the (fixed as above) old method for a test input at h1 = 0 then I think the delta is circa 1e-6 which is acceptable I think.

Question is what to do for -11/-10. I think you could backport this fix or at least use the midpoint and come up with a way to handle the h1>0 case.

[codecov-commenter]

Codecov Report

Merging #50 (4f46929) into master (f751918) will not change coverage.
The diff coverage is 100.00%.

@@           Coverage Diff           @@
##           master      #50   +/-   ##
=======================================
  Coverage   96.61%   96.61%           
=======================================
  Files          25       25           
  Lines        6672     6672           
=======================================
  Hits         6446     6446           
  Misses        226      226           

Impacted Files	Coverage Δ
itur/models/itu676.py	98.44% <100.00%> (ø)

---

Continue to review full report at Codecov.

Legend - Click here to learn more
Δ = absolute <relative> (impact), ø = not affected, ? = missing data
Powered by Codecov. Last update f751918...4f46929. Read the comment docs.