Implement common analysis data value object.

Implement common analysis data value object.

Makes handling of analysis data values simpler and more uniform.
Mainly changed internal representation of the data values in the
analysisdata module, and added extra methods to access those directly
from the public interface in dataframe.h.  It should be considered
whether old access methods (y(), dy(), present()) are needed any longer,
but this may wait.

Related to issue #827.

Change-Id: I4b12b4cd12de9a874e0ff4d3267ff779360a0f7a