created self.warnings outside except block
Merge pull request #831 from eteq/fix-automodsumm-warning

automodsumm: created self.warnings at start of run
Merge pull request #858 from mdboom/utils/check-file-size-before-download

Check free space before downloading a file
Merge pull request #876 from mdboom/unit/arcsec-prefixes

Units: SI prefixes for angular units
Merge pull request #863 from mdboom/unit/dont_compose_to_degrees

units: use "radian" for si/cgs units for angle?
Merge pull request #857 from mdboom/utils/delete-file-when-download-fails

Delete the temporary file if a download fails or is interrupted
Based an idea by @adrn in #794, this adds "automatic" members to
Quantity that allow the user to easily convert to other units.

This involved add a more complex UnitRegistry class, that also keeps
a dictionary of all registered units sorted by their physical
type.

From this new registry, a `compose` can also limit the set of units
that it needs to search through for a match, making it faster, so
this PR has the added bonus of being a performance improvement.
Update docs
Updates from comments in PR and other minor doc changes etc.
Fix typo