Jonathan/fix filter

Description

This branch updates the tests so that they catch that bst checkout should not redirect from filter elements, and stops filter elements from redirecting.

Changes proposed in this merge request:

• A new PipelineSelection mode, REDIRECT, has been added.
• The bst track command's --deps argument gains the option "redirect", which is now the default, instead of None, so the default behaviour of tracking is still to redirect where possible.
• Stream's workspace_open, workspace_reset, and redirect_element_names specify that they call Stream._load() with PipelineSelection.REDIRECT.

This merge request, when approved, will close issue/bug: #457 (closed)

CHANGELOG/Release Notes