Create merge notebooks

[eoydvin]

This PR adds the example notebook for merging CML and PWS data with weather radar. So far I have not done any QC of CML or PWS data.

The PR changes the following existing files:

• 1_data_preparation.ipynb
  • Move the OpenMRG transformation (originally from sandbox) outside OpenMRG+ repository. For reference this is now done in this fork, there is no plan to merge this fork. The OpenMRG+ data will use the transformed files directly.
• 2_cml_processing.ipynb
  • CML processing is moved inside examples_merge.ipynb
• 3_adjust_radar.ipynb
  • Merging examples and plotting is moved inside examples_merge.ipynb

[eoydvin]

Note that currently this PR uses mergeplg from my fork to use the most recent changes, which should just be capping max estimates at 150 mm/h