Core: provide double precision getters or change APIs

[GPMueller]

I did not realize that float had such horrible precision... Only 7 digits https://stackoverflow.com/a/3498243/4069571
chain.get_energy for example will therefore return extremely imprecise results.

This should be changed either in the APIs, or by adding API functions which return doubles.

[GPMueller]

The only drawback of using the scalar alias would be that the web-UI that goes through the C API would have to be built with single precision, because the API needs to be single precision.

Currently, @MSallermann and I agree that that would be the better tradeoff vs both of

• duplicating most of the APIs
• limiting the APIs to single precision

[GPMueller]

Fixed by PR #628, which was merged into develop with becc531