added removeFromGroupByCode() function to Member.php

I was using the addToGroupByCode() function today and it would have been handy to have a removeFromGroupByCode() so I decided to add one.
Added a unit test for removeFromGroupByCode()

Added a unit test for removeFromGroupByCode()
Updated group filter

Updated group filter
make setHeadingLevel a chainable method
Merge pull request #2634 from pine3ree/patch-1

make setHeadingLevel a chainable method
Merge pull request #2608 from andrewvt/3.1

added removeFromGroupByCode() function to Member.php