Enabled templates by geometry type

[flippmoke]

Possible solution for #27
/cc @kkaefer @jfirebaugh

[jfirebaugh]

Can we do this in a way that preserves the existing API with a default type of double?

[flippmoke]

@jfirebaugh will that work?

[jfirebaugh]

No, I mean make this a change that does not require any changes to the tests.

Supporting types other than double is a niche case, and technically outside of the GeoJSON spec. So we should preserve the types and function call syntax for for the existing, common case, avoiding breaking existing code, and introduce new calls and types for this added, uncommon case.

[flippmoke]

@jfirebaugh as time goes on, I more and more would love to be able to support all types for encoding and decoding. I know this is an uncommon case for most, but in general I find it very nice if all our tools support all the geometry.hpp types possible. I would love to revisit this pull request and provide general support for all reasonable types.