Add support for Handle const assignment from non const instances
This MR builds on !218 (merged) as it wasn't really practical to disentangle the two.
This MR adds support for copy assigning or constructing a XYZHandle<const T>
from a XYZHandle<T>
.
I also extended the test MyGaudiAlg.py
test in GaudiExamples to includes tests for this feature and ToolHandle<const T>
support in general.
Developed due to the discussion in !215 (merged)