(core) Fix imports into reference columns, and support two ways to import Numeric as a reference.

Summary:
- When importing into a Ref column, use lookupOne() formula for correct previews.
- When selecting columns to import into a Ref column, now a Numeric column like
  'Order' will produce two options: "Order" and "Order (as row ID)".
- Fixes exports to correct the formatting of visible columns. This addresses multiple bugs:
  1. Formatting wasn't used, e.g. a Ref showing a custom-formatted date was still presented as YYYY-MM-DD in CSVs.
  2. Ref showing a Numeric column was formatted as if a row ID (e.g. `Table1[1.5]`), which is very wrong.
- If importing into a table that doesn't have a primary view, don't switch page after import.

Refactorings:
- Generalize GenImporterView to be usable in more cases; removed near-duplicated logic from node side
- Some other refactoring in importing code.
- Fix field/column option selection in ValueParser
- Add NUM() helper to turn integer-valued floats into ints, useful for "as row ID" lookups.

Test Plan: Added test cases for imports into reference columns, updated Exports test fixtures.

Reviewers: georgegevoian

Reviewed By: georgegevoian

Differential Revision: https://phab.getgrist.com/D3875

sandbox/grist/useractions.py

@@ -2134,13 +2134,6 @@ class UserActions(object):
   #----------------------------------------------------------------------
 
   @useraction
-  def GenImporterView(self, source_table_id, dest_table_id, transform_rule = None):
-    return self._import_actions.DoGenImporterView(source_table_id, dest_table_id, transform_rule)
-
-  @useraction
-  def MakeImportTransformColumns(self, source_table_id, transform_rule, gen_all):
-    return self._import_actions.MakeImportTransformColumns(source_table_id, transform_rule, gen_all)
-
-  @useraction
-  def FillTransformRuleColIds(self, transform_rule):
-    return self._import_actions.FillTransformRuleColIds(transform_rule)
+  def GenImporterView(self, source_table_id, dest_table_id, transform_rule=None, options=None):
+    return self._import_actions.DoGenImporterView(
+        source_table_id, dest_table_id, transform_rule, options or {})