Add support for sre/distribution summary methods (including merge), advanced receivin...

Add support for sre/distribution summary methods (including merge), advanced receiving, and unit-less receiving in the serial control items tab

This commit includes three new minor features for the Serial Control interface.  First, you are now able to set the latent 'summary_method' field on distributions, with the most significant new setting being 'merge with sre'.  This setting effectively combines the sre marc with the generated holdings to form a single unified statement.  Second, the previous 'receive' functionality has been renamed to 'advanced receive', and 'receive' will now show items from an entire subscription, but not allow you to receive directly into a specific unit (you can still receive into new/auto units).  Third, a basic menu entry has been added for receving into 'no unit', allowing items to be received not forcing them to be unitized.

git-svn-id: svn://svn.open-ils.org/ILS/branches/rel_2_1@20379 dcc99617-32d9-48b4-a31d-7c20da2025e4