rising_sun: model architecture

[lhaze]

Prepare an architecture & a skeleton code of models:

• [x] load/save some instances of models to DB
• [x] load some instances of models from configuration files (YAML)
• [x] prepare test process (doctests/pytest)
• [x] instance register for the model instances loaded from the configuration
• [x] supply some validation for models
• [x] prepare an entry point for library (all models loaded, main code loaded)

[lhaze]

Questions: a) Does validation of model should block building a model (__init__) with invalid state? b) Or should I weaken default validation in some manner? c) Should the validation process be fired only by explicitly calling is_valid/validate?

[lhaze]

Done.