`aggregation_control` is documented as optional but coded as mandatory in pydantic model

Description

The esm-catalog-spec.md documentation says that the aggregation_control field is optional

intake-esm/docs/source/reference/esm-catalog-spec.md

Line 72 in c7cc334

    
           | aggregation_control | [Aggregation Control Object](#aggregation-control-object) | **OPTIONAL.** Description of how to support aggregation of multiple assets into a single xarray data set.                                                              |

but the pydantic model defines is as mandatory

intake-esm/intake_esm/cat.py

Line 114 in c7cc334

aggregation_control: AggregationControl

Thanks for pointing this out @huard - I added a PR (#551) to make sure that this is fixed.