The format method can accept a format specifier name, and calls that formatter instead.
If the specifier does not correspond to a valid unit formatter, then the generic string version is used,
and the format code applied to that generic string.

get_format would fail when a format didn't exist, as __all__ contained the function get_format itself.
This squashes that bug using inspect.isclass instead of a cludgy slice of __all__

Thanks to @mdbooms suggestion, I realized I fixed a single bug, but
added some poor code for others. This relies on error catching in
__format__ to determine if a specified format exists or not, instead
of a new `has_format` method

Units can use format codes in new-style format strings to set the desired formatting builder.

It is a list of formats that can be found by get_format()

(which didn't work)

object

Add a "show_in_browser" feature for tables

Rebased in eteq/expand-flux-conv to clean up commit log a bit

…n-doc-issue-1567

Remove incorrect description of setting a configuration value at runtime

update nduncertainty for multiple/divide

…anual merge

…ges to units.quantity and units.astrophys packages.