investigate use of BaseFieldDefinition

Problem/Motivation

This looks iffy in hook_entity_bundle_field_info_alter() -- *BUNDLE*!

      $fields[$field_name] = BaseFieldDefinition::create('date_occur_computed')

Computed Field module has this to say about it:

 * @todo When https://www.drupal.org/node/2346347 is fixed, this should be split
 * between this class and a new class that inherits from BundleFieldDefinition.

Needs looking into.

Steps to reproduce

Proposed resolution

Remaining tasks

User interface changes

API changes

Data model changes