**Use case**

Generate models once and reuse them.

**Describe the solution you'd like**

Add command line options:
```
--save file - write serialized models on disk after training and before generating the data; the user can set --limit to 0 to only train the models and save them without proceeding to data generation;
--load file - instead of training the models, load the prepared models from the file and start generating the data.
```

Serialization format: one byte for version (equals to 0), then simple binary serialization of all the models; the whole file is compressed with ZSTD level 1 using CompressedWriteBuffer (no need to allow any compression options to choose).
{
        if (params.num_buckets_cutoff)
        {
            for (auto & elem : table)
            {
                Histogram & histogram = elem.getMapped();
                if (histogram.buckets.size() < params.num_buckets_cutoff)
                {
                    histogram.buckets.clear();
                    histogram.total = 0;
                }
            }
        }