Use compressed numpy format by default in compressed mode (next major release)

[mverleg]

Numpy arrays are stored as text arrays by default. This is readable but takes a lot of space.

When the file is compressed, it perhaps makes more sense to use the new compact format introduced in #9.

Since this is a backwards-incompatible change, it will have to wait until the next major version (4.0.0).

For now the compressed format can be used like this (compression is optional):

json = dumps(data, compression=True, properties={'ndarray_compact': True})

There's also a warning announcing this change, which you can silence by explicitly disabling the new format:

json = dumps(data, properties={'ndarray_compact': False})

[mverleg]

It might be important to support F-order arrays before this becomes default