Support FlowElements in MET Reconstruction

This adds support for FlowElements in the main MET reconstruction algorithms. This almost entirely affects the creation of the MET associations and maps, as opposed to analysis-level tools such as METMaker. The main change from the user point of view is that METAssociators now have an attribute "FlowElementCollection" which, if configured to a non-empty string, will override the normal input PFO collection with the specified FlowElement collection.

The (old) config has been updated accordingly, there is now a new config flag metFlags.UseFlowElements that will hand the FE collection to the METAssociators instead of the PFO collection if set to True (it's False by default).

I've also removed some obsolete code as well: RetrievePFOTool and the PFO handling in METSoftTermsTool, which is no longer in use except for building basic cluster/track-based MET for monitoring purposes.

Some of the changes also include fixing indentations which previously used inconsistently-mixed tabs and spaces such that everything is now uniform (apologies to the reviewer for that - developing this was going to be a pain without fixing it!)

Tagging @khoo, @williams, @cdelitzs, @mswiatlo, @hpacey, @maklein, @sawyer