Improve basic functions and extend readme example

[lupemba]

This PR contains a number of smaller changes.

• Rename read_dataset to open_dataset since the method just opens the dataset and only reads the necessary meta data.
• Add maskingvalue argument to open_dataset. I often put it to NaN in my workflows so users are probably also going to need it.
• Add as_array method so we can remove from juliacall import Main from the example.
• Add close_dataset method
• Add shape method to wrap Main.size. The Main.size function is much more general in julia compared to directly accessing the .size property of a Julia array. E.g. Main.size works for all julia array types and not just the standard one.
• Update README example to include new methods.
• Add a small section with np.array to the example.

[pacospace]

Thanks! LGTM