BUG: Fixed exclusion list

Currently iris test coding standards will perform pep8 checks on files
contained within the build directory and then ignore the results.  This
commit removes this checking entirely and fixes a bug present in
generating the assumed directory for the iris repository from the iris
path (a problem which arises when considering softlink vs real paths).
Add PP load + save support for potential-temperature vertical levels.
Review changes.
Add PartialDateTime
Experimental regrid.
Unicode tests updated, new reference string files added
Review actions: comment added, sys module introduced now explicitly removed at contextmgr exit
Resolve PP save rule bug.
Future context restores state even when interrupted.
Moved TypeError tests per PR discussion