Sigma renamed to Err for centroids, fluxes and aperture corrections

As of DM-15244 and in aid of RFC-333 all existing afw table fields that had suffix Sigma now have suffix Err. This includes centroids, fluxes and aperture corrections. I also changed argument and variable names for consistency.

Existing persisted tables can still be read: for each field whose name ends in Sigma an alias is made that ends in Err. In order to implement this, I incremented the VERSION constant in afw/table/SchemaImpl.h from 1 to 2.

There was a typo in the aperture correction code that caused ci_hsc to fail its tests overnight. This has been fixed in DM-15355.

Note: any tables containing aperture corrections written using the daily built last night or using the master branch from the DM-15244 merge until about 2018-08-08 05:45 PDT will almost certainly not read correctly using earlier or later Science Pipelines software. Hopefully this does not present a problem.