Refactor groupby helper from tempita to fused types
Refactoring groupby_helper to use fused types instead of tempita for functions such as group_add.

@jbrockmendel if this is what you meant in the previous [PR](https://github.com/pandas-dev/pandas/pull/24932#discussion_r251094683) I'll do the refactoring for the rest of the functions in groupby_helper.pxi.in.