Develop

**PLEASE IGNORE THE UPDATES in**: 
- `src/test/`
- `examples/spouse_example/`
- `src/main/resources/logback.xml`
(will remove them in the next commit)

Updates:
- Added Dimmwitted sampler
- In `SQLInferenceDataStore.scala`:
  - Fix some bugs in using || to concat strings: strings should be quoted by '' to prevent recognized as column name.
  - Use a dummy way to prevent cases that factor_id = null, where last query table is empty.