Made echo function publicly available in the Script interface.  Instead of adding a requested comment function to the Script interface or the LoggingScript, we decided to add the echo functionality instead.  Note that this function should not have any negative effect since it is mostly implemented as a no-op.  Only the LoggingScript actually does something more than returning the input.
Fixed endCheck notification

The notification was missing in all cases in DPLLEngine.solve.  This lead to bad performance and even worse interpolants in many cases.