**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).
{
private:
    UInt64 seed;
public:
    explicit UnsignedIntegerModel(UInt64 seed_) : seed(seed_) {}
    void train(const IColumn &) override {}
    void finalize() override {}
    ColumnPtr generate(const IColumn & column) override
    {
        MutableColumnPtr res = column.cloneEmpty();
        size_t size = column.size();
        res->reserve(size);
        for (size_t i = 0; i < size; ++i)
            res->insert(transform(column.getUInt(i), seed));
        return res;
    }
    void updateSeed() override
    {
        seed = hash(seed);
    }
};
static Int64 transformSigned(Int64 x, UInt64 seed)
{
    if (x >= 0)
        return transform(x, seed);
    else
        return -transform(-x, seed);    
}
class SignedIntegerModel : public IModel