Abstract Base Class for all ROM constructors

[robertvava]

This PR incorporates and abstract base class (ABC) called DecompositionModel for all ROM constructors (currently TruncatedSVD and OptDMD). DecompositionModel enforces two abstract methods: fit() and reconstruct(). It also contains a method _check_is_fitted() to check if a ROM has been fitted prior to reconstructing or forecasting.

Additionally, this PR replaces logger.exception() with logger.error() where an error is raised outside an exception handler. This is best practice (see here).

Closes #24.

[dsj976]

Thanks @robertvava, this is great. I have made some minor changes.

[dsj976]

@robertvava this needs a last review by you before it can get merged, as I was the last pusher.

[robertvava]

Thanks for the changes, looks great!