Fix Bug - Exceptions when calling annotate without a Context Store

#### Problem
Exceptions when calling annotate without `context.mem` in the models

#### Relevant Changes
  - As suggested created `NoContextSimilarity` class which outputs default context similarity values
  - SpotlightModel creates `NoContextSimilarity` instance if it can find `context.mem`, otherwise it uses `GenerativeContextSimilarity`

- [ ] [Sehr Geehrte ;) ] @jodaiber