LinearWeighted::standard_error

[Peter]

Check that implementation match specification (docs in base class).

Especially the following design criteria should be fullfilled:

• Setting all weights to unity yields the same result as the non-weighted version.
• Rescaling the weights does not change any function.
• Setting a weight to zero is equivalent to removing the data point.

daughter ticket of #81

[Jari Häkkinen]

Milestone Regression redesign deleted

[Peter]

(In [730]) fixes #167 and #160