Include computation of scale factor (k) and convergence angle (γ)

[yongjun21]

I used this library not for lat/lon to UTM x/y conversion but for deriving the transformation from UTM x/y offset to Web Mercator x/y offset from a ref point.

Current process involves

1. converting 3 nearby points in UTM to lat/lon then
2. calculating the Web Mercator meter offsets of the 3 transformed points then
3. solving the linear equations to get the transformation that maps UTM offsets (from a ref point) to Web Mercator offsets
4. the result is only an approximation since it relies on solving the transformation from the empirical values of 3 transformed points

After digging into the theory behind UTM, I realize this can be done more simply and accurately but computing the convergence angle and scale factor for the ref point: https://en.wikipedia.org/wiki/Transverse_Mercator_projection#convergence.

Like most other UTM libs, this does not include functions to compute k and γ but is probably not hard to extend since most of the logic for computing preliminary and intermediate values are already inside.

If maintainer is not able to commit time, I might be able to create a PR for this

[bartvanandel]

For completeness, Wikipedia links for both formulas:

• Point scale ($k$)
• Convergence ($γ$)

If you're still interested, a PR would be nice!