Restricting number of times a specific child level can be claimed

[dparker1005]

Is your feature request related to a problem? Please describe.
Some websites may want to limit the amount of time each "child" level can be claimed for any given group code.

Ex: Someone purchases a group which allows one "spouse level" group to be claimed, and unlimited "child levels" to be claimed.

Describe the solution you'd like
Currently, this would require custom code hooking into pmpro_registration_checks that checks if a group code is being used and, if it is, looks through the existing members in the group to check if there is "room" for claiming the level being purchased with the given group.