Qpimage

- enh: add QPImage meta data keys "angle", "focus", "device",
and "software"

- fix: strip third axis from input hologram images

- fix: subclass MetaDict from collections.UserDict instead of dict
(TypeError super() argument 1 must be type, not dict)
- docs: fix rtd build

- fix: easy fix for regression in h5py with backing_store=False

- feat: allow to set hologram filter size in Fourier indices
(Fourier space pixels) instead of relative to the distance
between central band and side band by setting the argument
"filter_size_interpretation" to "frequency index"
- fix: Fourier filter size for holograms is now inclusive,
i.e. points *on* the perimeter of the filter are included
- setup: bumpy nrefocus from 0.2.1 to 0.4.3 (use new interface)
- ref: remove IMAGE_* attributes from HDF5 datasets, because they
anyway only work with uin8 data, and not with float data
- ref: code cleanup

- setup: bump nrefocus from 0.2.0 to 0.2.1