Add ReaderOptions and ReaderStreamedOptions

[nilgoyette]

• Make InMemNiftiObject's methods privates
• Make StreamedNiftiObject's methods privates
• Remove the deprecated new_from_stream method
• Apply one fix on header when requested
• Fix all tests

Sorry, there was some automatic formatting when I saved (like in lib.rs). The other concerns I have at the moment are:

• One can still read the complete image by creating a File, reading the header, then reading the volume. Should I block that? By making some methods pub crate in header.rs and volume/inmen.rs?
• I don't want to block it too much because we want a way to read only the header without reading the complete image. We're already using this in our codebase. It's practical and much faster.
• I separated the features in ReaderOptions and ReaderStreamedOptions to avoid repetitive names read_file_streamed/_rank/_pair/_pair_rank (I can at least remove the 'streamed' in all names. But it does create some code copy. If we ever add some general methods, like read_header, the code will be exactly the same. So, I'm not sure it was the right decision.

[Enet4]

Thank you for working on this @nilgoyette. Please see the comments, the most important concerns are (1) whether to keep former public APIs and (2) document the concrete header fixes.

[Enet4]

👍 Thanks!