Improve function naming in volume-related functions

[Enet4]

The naming guidelines established in the Rust API guidelines help us create interfaces that are generally intuitive and non-confusing for the usual Rust programmer. During the development of nifti, I accidentally gave some methods and functions unconventional names, which I believe should be corrected before the crate enters a mature state. To make the migration process smoother, I deprecated the old methods for removal, starting from 0.6.0, and will only remove them in 0.7.0.

• deprecate InMemNiftiVolume::to_raw_data into into_raw_data
• deprecate InMemNiftiVolume::get_raw_data and InMemNiftiVolume::get_raw_data_mut into InMemNiftiVolume::raw_data and InMemNiftiVolume::raw_data_mut
• deprecate IntoNdarray::to_ndarray into into_ndarray
• rename util function to_img_file_gz to into_img_file_gz

[nilgoyette]

It's always a good idea to be consistant! LGTM.

[Enet4]

Thanks for having a look, @nilgoyette. :+1: