Add explainer replicas and handle scale to zero

[ukclivecox]

What this PR does / why we need it:

• Scales explainer to zero if main predictor replicas is zero
• Allows explainer replicas to be add. New addition to CRD.

Which issue(s) this PR fixes:

Fixes #3241

Special notes for your reviewer:

Does this PR introduce a user-facing change?:

[review-notebook-app]

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[ukclivecox]

/test notebooks

[ukclivecox]

/test integration

[axsaucedo]

Added a suggestion on test, but other than that looks good

[axsaucedo]

/test integration

[axsaucedo]

/test notebooks

[ukclivecox]

/test integration

[ukclivecox]

/test notebooks

[axsaucedo]

/test integration

[ukclivecox]

/test integration

[axsaucedo]

/test integration

[axsaucedo]

Merge conflict needs to be resolved to rerun tests

[axsaucedo]

/test integration

[axsaucedo]

/test integration

[axsaucedo]

/test notebooks

[axsaucedo]

Notebooks and integration tests pass, and all comments resolved, merging