Merge values control

[jamesscottbrown]

What does this change?

This adds a component that allows users to define how different values of a categorical variable should be merged into new categories.

Why?

This was needed in th emultivariate census explorer, and will be needed for the LBSM tool too.

Does this introduce new dependencies?

No.

Is it complete?

• [x] Have you included changeset file?
• [x] If this adds a new component, is it exported via index.js?