Small bug fixes in tests

[odunbar]

Purpose

• Closes Small test bugs #265
• Addresses constrained_gaussian freezes #261 (1/2)
  Leads to a more robust test pipeline and constructor constrained_gaussian

Content

• fixes bugs within Small test bugs #265 ,
• resolves an eks object bug where it would mix prior and final ensemble, and now puts plots in the EKP test subdirectory
• adds warning for when users prescribe values in constrained_gaussian which will be slow to solve. In these cases a large amount of mass will be concentrated on the boundary.
• tightens error of termination to mu + sig < upper_bound (before was mu + 0.7*sig > 1 etc.) likewise for lower bound

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• I have read and checked the items on the review checklist.

[odunbar]

Note - this small change primarily affects unit testing, and there are no user interface changes either, so I shall merge on test success.

[odunbar]

bors r+

[bors]

Build succeeded:

• docs-build
• format
• test_success