Fixes #1881

Digging into the implementation of proj=cea, it appears that
k_0 and lat_ts are intended to be exclusive ways of specifying the
same concept. EPSG only models the variant using lat_s.
So if k_0 is found and lat_ts is absent, compute the equivalent
value of lat_ts from k_0.

Note: k_0 should normally be in the [0,1] range. In case creative users
would use something outside, we raise an exception, even if the cea
implementation could potentially deal with any k_0 value. Hopefully
this is a (reasonable) limitation that will address nominal use cases.

Previous version was describing oea by mistake. It is also a projection
by Snyder and the isea docs was based on the paper on the oea
projection. This commit fixes that.

This is a super confusing topic, but from experiments, and in particular with
the example of Fairgrieve projection, o_lat_p and o_lon_p are the coordinates
of the North pole of the unrotated geographic CRS expressed in the rotated
geographic CRS.
This is different from defining a 'new pole', which would be the coordinates
of the rotated North pole expressed in the unrotated CRS.