Update notebook to add more variables and GPM IMERG dataset

[abarciauskas-bgse]

This updates the notebook to use a json file to generate the STAC json with the appropriate parameters, specifically for the renders object.

A few things need to be updated in the future:

• time_bnds should not show up for GPM IMERG as it's not a data variable time_bnds should not show up for GPM IMERG as it's not a data variable #15
• the other variables (such as the _cnt variables) in the GPM IMERG dataset are throwing an error when I try and create tiles from them (dst_nodata must be in valid range for destination dtype) from titiler-xarray so I haven't included renders options for those variables yet dst_nodata must be in valid range for destination dtype titiler-multidim#50
• the MUR SST mask variable is a categorical variable, so I need to think/investigate how to add appropriate renders object for categorical variables. Add MUR SST mask variable renders parameters #16