Feature/write complex valued data

[twitzelbos]

Okay, here it goes:

• reads and writes RGB, RGBA, Complex64 and Complex128 now

• the new (additional) tests write RGB and RGBA using write_nifti

• I left the old write_rgb_nifti in place, but it can be removed

• I made a bit of a mess replacing LinearTransform with NiftiDataRescaler (I did not like the name LinearTransform), resulting in more code, but could clean this up more.

• For reading there is a function into_nifti_typed_data() now, which provides a vec of the native type, for when an ndarray is not needed/wanted

• resolves issue #105

[twitzelbos]

This is ready for review, @nilgoyette @Enet4

[twitzelbos]

Would you like me to change/add test cases, or is this enough for now? In principle, we could make tests where we write matrices of random content, read them back, and compare, but I feel the bitwise comparison to independently generated files is a better test. @nilgoyette @Enet4

[nilgoyette]

LGTM. Thank you for your work!