Updating latest release

Mid-term release.

Version 0.7

In this release there are improvements on several aspects:

• Fixed a long term docs issue
• Updated to be compatible with Astropy >= 4.
• Deprecated pixeldata property, and moved to data, simpler and shorter
• License is now consistent and updated to follow a BSD3

Version 0.6

In this release:

• Versioning inside package

>>> import properimage
>>> properimage.__version__
'0.6'

• Only pip installation recommended
• Quick usage in README