Fix error when using a template at different resolution to regrid a restart file

[1Dandan]

Name and Institution (Required)

Name: Dandan Zhang
Institution: Harvard University

Describe the update

I need to regrid restart file to a resolution that may not be available at the resolution of current generated restarts files.
As the template for a restarting file is only to add attributes, it should be benign to use templates at different resolutions.
Thus,

• I removed the part of raising error when the template resolution does not match. Instead, I cleaned global attributes from the template and only add attributes when stretched GCHP is used.
• I also update the drop to be drop_vars as drop is deprecated in newer xarray version.

Expected changes

It will allow regridding restart file to any spatial resolution regardless the resolution of the template restart file.

Reference(s)

N/A

[yantosca]

Thanks @1Dandan. Looks good to merge!