[PROJ] "eck_greif" projection, "libproject"?

[Even Rouault]

> And when comparing hammer.cpp with EckertGreifendorffProjection.java, it
> seems Eckert-Greifendorff is actually just "+proj=hammer +W=0.25 +m=1"

Correction: should read '+M=' uppercase (but as M=1 is the default value, that 
won't change anything to the result):

+proj=hammer +W=0.25 +M=1

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