Force score expressions to be elementwise

[isaisabel]

Currently, score expressions are evaluated by converting the scores on a layer into a flat matrix. For example, a layer with two annotated techniques with the scores 42, 51 would be converted into [42, 51]. The score-expression evaluation is performed on this matrix. This leads to confusion since the requirement for elementwise operators (e.g .* instead of *) is not intuitive (#79).

Rather than documenting this behavior as proposed by #80, instead we should change the way things are calculated to be elementwise by default. This will lead to a more intuitive experience and also allow for expressions like conditionals (e.g a > 40? a : b) which are currently not possible.

[isaisabel]

Note: closing #80 since this issue makes it irrelevant.

[isaisabel]

Implemented in feature/elementwise-score-ops and merged into develop.