Simplify API of GenesisGroup.purchase
The API of `GenesisGroup.purchase` could be simplified by not including the `value` parameter that is required to be equivalent to msg.value:\\n```\\nrequire(msg.value == value, "GenesisGroup: value mismatch");\\n```\\n\\nUsing `msg.value` might make the API more explicit and avoid requiring `msg.value == value`. It can also save some gas due to fewer inputs and fewer checks.
Consider dropping the `value` parameter and changing the code to use `msg.value` instead.
```\\nrequire(msg.value == value, "GenesisGroup: value mismatch");\\n```\\n