create a simple cache for storing model hierarchy lookups
create simple methods in a base class for model hierarchy navigation
create a wrapper for hierarchy navigation using different types
adjust PojosImpl to use hierarchy navigator

this will make changes for ticket 11019 easy to implement
change preprocessor to use hierarchy navigator
adjust do-all for change in preprocessor constructor
update preprocessor test class with latest changes
remove weird rebase artifact
replace local function with third-party library function
Removing imports reported as unused
Remove more unused imports
Temporarily revert changes to webadmin files to avoid conflict with #2127
Re-add imports still required in dev_5_0
Check for duplicate pixels IDs when adding to list.
Remove unused variables

When unused variables had previously been used in now commented out code,
they were commented out as well instead of completely removed.
Remove commented out code
Remove commented out code
Merge pull request #2266 from knabar/flake8-cleanup-unused-vars-dev50

omeroweb flake8 cleanup: Remove unused variables (dev_5_0)
Remove commented out import
Merge pull request #2158 from mtbc/trac-11091-hierarchy-navigation

assist #11091: refactor server DB queries for hierarchy navigation (rebased)