GeoScale should not inherit from Scale

[martinRenou]

In d3, GeoProjections are not considered being scales. Maybe it should not inherit from the Scale, or it should share less logic with Scales in order to compile correctly (e.g. a GeoProjection does not have a range method).

[SylvainCorlay]

Indeed, although it was an intentional inheritance.

Basically, d3.js does not support N-D scale, (2-D mappings that cannot be separated into x and y scales), although d3 projections are essentially just that and when we made the bqplot maps, we used the same patterns with projections as we did with scales in other marks.