Fix ticket #96 add Extent to CRS

[mukoki]

Proposition to add extent to CRS. As Extent is an interface, I just implemented it in CoordinateReferenceSystem. Default implementation always return true. GeodeticCRS implementation makes it possible to define Extent from projected coordinates or from geographic coordinates (referring to this CRS's Datum). In the later case, geographic coordinates are transformed into this CRS's coordinates. Needs more tests.

[ebocher]

Dear @mukoki We plan to release a new version of CTS. Should this PR be included in the next release? Cheers

[mukoki]

Quite old one... should still be OK. It would probably need more tests though. I think that merging it just before a release does not make much sense as it is not sufficient to solve concrete problems. I would recommand to merge it, test it, try to add new capabilities using the new methods then release.

[ebocher]

Thanks I'll add it to the 1.7 road map