[MS-37913] Add elementary data-partition group validation in remove api

As per #162 , we are now blocking the removal of users from the elementary data partition group unless it's the last group the user is a member of.

Changes include:

1. Delete member API: now deletes a user from all other groups first, and in the end it deletes from users group
2. Remove member API: checks the number of groups a user is directly a child of, if its more than one we're rejecting the call asking the user to use the Delete member api
3. Minor refactoring
4. Documentation