Darjeeling

* added (sub-optimal) code to ensure that all provisioned Docker containers are destroyed when the search is interrupted.
* modified SampleByLocalization to use sample method provided by BugZoo.

* minor improvements to logging format
* reimplemented "terminate_early" functionality

* added snippet origin restriction to line-snippet generators (i.e., when synthesising code, only use code that came from the particular file that is being repaired; follows standard APR practices.)

* renamed donor pools to snippet databases
* removed existing donor pool construction code and replaced with a simpler, more effective alternative in the Problem class -- only snippets that are executed by the program-under-repair are considered for membership
* added hard-coded snippet filters to Problem class
* added __len__ to SnippetDatabase
* improvements to logging
* implemented several compositional generators for lazily constructing patches (and transformations) according to certain policies
* reduced memory footprint for Problem instances
* pinned to BugZoo 2.0.8