Fix the omerc-bb area to use a sphere as ellps

[mraspaud]

This fixes problems at the poles with folding of the omerc-based areas.
See OSGeo/PROJ#1922 for more details.

• Helps with Resampling of long passes of metop l1b eps data gives strange results #1002
• Tests passed

[coveralls]

Coverage remained the same at 88.857% when pulling 9c5dad7 on mraspaud:fix-omerc-fold into eae41fa on pytroll:master.

[codecov]

Codecov Report

Merging #1072 into master will not change coverage by %.
The diff coverage is n/a.

@@           Coverage Diff           @@
##           master    #1072   +/-   ##
=======================================
  Coverage   88.85%   88.85%           
=======================================
  Files         192      192           
  Lines       29517    29517           
=======================================
  Hits        26228    26228           
  Misses       3289     3289           

---

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 eae41fa...9c5dad7. Read the comment docs.

[TAlonglong]

Hm, not sure I like it, but I guess this a limitation in the implementation of the omerc, so for long passes this is needed.

[TAlonglong]

LGMT