This initial commit changes the Summary class's API for consuming
block-averaged well level pressure values (summary keywords WBPn).
The former approach was intended to consume a collection of source
values--pressures, densities, and pore-volumes--and then to defer
calculation of the WBPn summary vectors to the Summary class.
This commit introduces a 'WellBlockAvgPress' container class which
holds precomputed WBPn results and the intention is to move the
calculation to the simulator side for greater parallelism.
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