getUniqueValues getter for Phase 2 Store Refactor

[jarmoza]

This PR closes #277 by implementing the following:

1. getUniqueValues getter in the store that takes every column linked to a given category and gathers unique values for each column via the dataTable. The getter outputs an object keyed on column with values that are an array of the unique values for that column. Missing values are skipped via lookup in the dataDictionary
2. getUniqueValues also takes an optional parameter maxValues to limit how many values are returned
3. displayItems in the annot-categorical component has been rewritten to accommodate the new output of getUniqueValues
4. Unit tests for this getter in store-getter-getUniqueValues.cy.js test the following scenarios: a) unique values for a single column, b) unique values for multiple columns, c) retrieving unique values with the optional maximum amount argument, and d) ensuring that no 'missing' values are returned with the unique values

[rmanaem]

Great work on the PR @jarmoza! The unit-test test is thorough and in-line comments are nice and helpful. I left two comments that simply address the type of uniqueValues mentioned in the in-line comments other than that It's good to go!