added pickling

[feldlime]

Description

Type of change

• [ ] Bug fix (non-breaking change which fixes an issue)
• [ ] New feature (non-breaking change which adds functionality)
• [ ] Breaking change (fix or feature that would cause existing functionality to not work as expected)
• [ ] Optimization

How Has This Been Tested?

Before submitting a PR, please check yourself against the following list. It would save us quite a lot of time.

• Have you read the contribution guide?
• Have you updated the relevant docstrings? We're using Numpy format, please double-check yourself
• Does your change require any new tests?
• Have you updated the changelog file?

[codecov[bot]]

Codecov Report

Attention: Patch coverage is 61.90476% with 8 lines in your changes missing coverage. Please review.

Please upload report for BASE (feature/model_saving@1d641fa). Learn more about missing BASE report.

Files with missing lines	Patch %	Lines
rectools/models/base.py	61.90%	8 Missing :warning:

Additional details and impacted files

```diff @@ Coverage Diff @@ ## feature/model_saving #195 +/- ## ======================================================= Coverage ? 99.77% ======================================================= Files ? 56 Lines ? 3540 Branches ? 0 ======================================================= Hits ? 3532 Misses ? 8 Partials ? 0 ```

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