Support serialisation to binary files

The method `void DTensor<T>::saveToFile(std::string pathToFile)` will become

```python
void DTensor<T>::saveToFile(std::string pathToFile, SerialisationType t)
```

where `SerialisationType` can be `SerialisationType.TEXT_ASCII` or `SerialisationType.BINARY` (enum). Alteratively, we could have different file extensions for text and binary files. 