Make rechit selection for cluster shape consistent with input clusters by construction.

This pull request depends on
https://github.com/cms-sw/cmssw/pull/1502
(and also the needed fix discussed in the comments there)

In order to produce fully consistent behaviour (only rechits belonging to the input cluster will be used, with full use of rechit fractions and no additional quality filtering)

Previous implementation did not guarantee consistent flag and severity level cleaning for clustering and shower shape computation.

@lgray, @argiro, @dbenedet, as discussed by mail earlier.