implement PlotPostfitShapes task

[mafrahm]

There still are some TODOs for a future PR:
- include data
- include correct uncertainty bands
- pass correct binning information

How to create postfit shapes:

1. Create your rebinned datacards (can be skipped since this is required from step 2)

law run hbw.ModifyDatacardsFlatRebin --version prod1 --inference-model rates_only

2. Get the command on how to run FitDiagnostics (this ensures that we keep the category name information when merging datacards)

law run hbw.PrepareInferenceTaskCalls --version prod1 --inference-model rates_only

3. Step 2 prints a task call. Copy-paste this and run this with Inference (needs a fresh shell). Run it again with --print-output 0 to get the output location of our FitDiagnostics file.

cd path/to/inference
source setup.sh
<insert_copied_task_call>
<insert_copied_task_call> --print-output 0

4. Run the PlotPostfitShapes task using the FitDiagnostics file from before. This task also knows some plotting parameters like yscale and shape-norm

law run hbw.PlotPostfitShapes --version prod1 --inference-model rates_only --fit-diagnostics-file <file_name>