Remove the is_improvement flag from Everest storage

[verveerpj]

The Everest storage currently stores an is_improvement flag with each batch. Some comments:

• It is only used by everviz, which is slated to be removed
• It stores a single flag per batch, although there may be multiple function results (see issue #10996 )
• It is based on the notion that the objective function improved, or, until recently for the optpp_q_newton algorithm, if the merit function improved. The latter option is being removed by PR #10963, which also adds filtering by constraint violations using a threshold approach. However, the latter is based on a ad-hoc threshold, which cannot be set by the user.

The concept of storing "best" or "improved" in storage is too limiting, this should be determined dynamically as needed, based on user input and the contents of the storage.

The is_improvement flag should be removed. Any visualization of "improvement", or "constraint violations" should be handled by the visualization tool, based on the contents of the storage.