GitLab

These changes elaborate the PR #700.

1. Python objects change list:
* `Individual` and `ParentOperator` classes are now frozen data classes;
* Every uid now normally exists in a single Individual object. This is a soft restriction, though, and any violation will be noticed with warning;
* `fitness` and `graph` of `Individual` now can be updated only once - during its evaluation;
* `Individual` has a new attribute called `native_generation`. It points to the number of generation in which the individual is obtained;
* `parent_operators` of individuals are not cleaned in every generation any more;
* `Individual.parent_operators` and `ParentOperator.parent_individuals` are now tuples;
* Side change: Moved `SingletonMeta` from `fedot.core.log` to `fedot.core.utilities.singleton_meta`
* Bug fix: `OptNode` now accepts `content` with `params`, not replacing parameters with default stub.
* Minor: `ind` argument of `mutate()` now has explicit name `individual`;
* Minor: many typing fixes.

2. `OptHitsory` serialization change list:
* Serialized `OptHistory` now has an attribute called `individuals_pool`. It contains all unique individuals from the history;
* In history JSON, `individuals` and `archive_history` now contain only uids that point to the elements of  `individuals_pool`;
* Any intermediate parent, that is not represented in any generation, is placed to `individuals_pool`. Its children point to it via its uid;
* As a bonus, newly-saved history JSONs now take much less size (about 10+ times lesser);
* All individuals not found in loaded histories now are re-created with the same uid. Their metadata contains an explicit message explaining that they are missing;
* Added support for relatively recent histories;
* Improved `OptHistory` tests to guarantee stability and backward compatibility;
* Updated the histories stored in Git repo.