348 update exists.eliminate() to reduce computations

[wdcraft01]

This branch provides a minor update to the Exists().eliminate() method, with the following 2 main changes:

• reinstate a small block of code (performing a generalization step) that had been previously commented-out. Although automation can handle the generalization step, some experimentation suggested that an explicit call without relying on automation can save some substantial search/computation effort;
• add a check and error message for cases when the eliminate() method is called prematurely when one or more of the Skolem constants still appear as a free variable in the targeted judgment.

The changes were tested with some simple examples in the existential demonstrations notebook but then the examples were deleted to keep this branch as clean as possible.