add AnaReentrantAlgorithm

Mostly a copy-and-paste of AnaAlgorithm, with cleanup and updates to inherit from AthReentrantAlgorithm.

Probably the main thing to look for for the reviewer are copy-and-paste errors.

Not sure if it was @tadej, @jburr, or @khoo who kept asking for this, but here it is. Please note that the systematics handles will need updates to work with reentrant algorithms (to say nothing of the ISystematicsTool).