Increase test coverage

The current percent of coverage is ~74%, which is relatively small and allows to make more thoughtless and untested changes. Even though some functionality can be covered with integration tests, it is not enough, since unit tests allow to easily track all inconsistencies. Covergae with units must be increased to make GOLEM more stable. So the current files with the smallest percent of coverage can be seen [here](https://app.codecov.io/gh/aimclub/GOLEM?search=&displayType=list). These are mainly:

- structural analysis
- graph builder, some visualizations
- bandits