[NEMO-460] Setting coders in CombinePerKey transformation (#303)
JIRA: [NEMO-460: Setting coders in CombinePerKey transformation](https://issues.apache.org/jira/projects/NEMO/issues/NEMO-460)
**Major changes:**
- Added the additional parameter "inputCoder" for GBKTransform constructor.
- Fixed the input coder and the output coder for the partial combine transform and the final combine transform.
**Minor changes to note:**
- Fixed the main output TupleTags for the partial combine transform and the final combine transform.
**Tests for the changes:**
- Current tests suffice.
**Other comments:**
- This needs to be merged after merging #302
Closes #303
