peirce_q: limit input to positive latitudes

Otherwise it returns junk (negative latitudes are mapped to
the same location as positive latitudes)

I'm a bit confused by PROJ peirce_q implementation. Looking at
the projection of the world, it looks like this matches the diamond
formulation of the right map shown at
https://desktop.arcgis.com/en/arcmap/latest/map/projections/peirce-quincuncial.htm,
but limited to the inner square of this diamond (which corresponds
to the northern hemisphere). We lack the 4 triangles on left, top,
right and bottom for the southern hemisphere. Furthermore, this
formulation of peirce_q does not seem to have the quincuncial
property of the square formulation (left images of the above ESRI
doc), or the one at https://en.wikipedia.org/wiki/Peirce_quincuncial_projection ...

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