Add pix2zphiRing, pix2zphiNest, ringAbove

[ziotom78]

No description provided.

[codecov]

Codecov Report

Merging #18 into master will increase coverage by 0.78%.
The diff coverage is 86.66%.

@@            Coverage Diff            @@
##           master     #18      +/-   ##
=========================================
+ Coverage   79.12%   79.9%   +0.78%     
=========================================
  Files          17      17              
  Lines         570     647      +77     
=========================================
+ Hits          451     517      +66     
- Misses        119     130      +11

Impacted Files	Coverage Δ
src/Healpix.jl	100% <ø> (ø)	⬆️
src/pixelfunc.jl	91.95% <86.66%> (-4.2%)	⬇️
src/interp.jl	100% <0%> (ø)	⬆️
src/math.jl	100% <0%> (ø)	⬆️
src/xyf.jl	92.85% <0%> (+0.54%)	⬆️
src/projections.jl	67.25% <0%> (+0.58%)	⬆️

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