Adding more options for data storage

[swkeemink]

Currently all extracted and separated data are stored as .npy files. A more common storage format would be HDF5, and for some user cases it might also be useful to not store anything at all.

I propose an optional argument like 'file_format', which can be 'npy', 'HDF5', or None.

This relates to #22 and might solve #81 .

[scottclowe]

I support this enhancement going ahead.

[swkeemink]

I have added the option for defining the data format as None, but still need to work on an hdf5 implementation. See pull request #86.

[scottclowe]

We already have an issue for adding HDF5 support: #22.

[swkeemink]

It wasn't written very well, but that one is for loading hdf5 files into FISSA containing ROIs or imaging data. However we should probably just combine them.