calibration: return only one struct (and the result)

[JonasVautherin]

Because of the Rx language bindings, we cannot have more than one return type (that is, one Result that says when there is an error and when the stream completes, and one actual struct containing data). This has to be reflected in the C++ API if we want to auto-generate those headers, too.

Moreover, some languages don't have the notion of NaN and default to 0 (which is an issue for e.g. a progress). For this reason, the resulting struct has booleans to explicitly say if a value is defined or not.

Finally, I tried to remove some code duplication (see here). It should behave the same in my understanding.