Add some more ESRI projection method mappings

[nyalldawson]

Adds

• Mercator_Variant_A
• Mercator_Variant_C
• Transverse_Cylindrical_Equal_Area

[rouault]

Suggested change

	# Eckert_Greifendorff: +proj=hammer +W=0.25 +m=1
	# Eckert_Greifendorff: +proj=hammer +W=0.25 +M=1

[rouault]

For the sake of being good open source citizens (and have an up-to-date reference), provided that Esri/projection-engine-db-doc#17 is merged, we should probably contribute updates to https://github.com/Esri/projection-engine-db-doc/blob/master/other/proj4-esri.txt, especially in the "PE projections not supported by PROJ4 " to make it smaller :-) Good candidates that we have already mapped are: Behrmann, Plate_Carree, Equidistant_Cylindrical_Ellipsoidal, Mercator_Auxiliary_Sphere

[nyalldawson]

There's quite a few "*" (only in libproject flag) which can be removed too...

[nyalldawson]

(besides, I'd find it kind of amusing if ESRI repos had me as a contributor 🙃)

[rouault]

I'd find it kind of amusing of ESRI repos had me as a contributor

ahah, be reassured, you're not alone: https://github.com/Esri/lerc/commits?author=rouault :-)