updated partial sums builder handle_named_function and handle_sum to …

[natalieb-noaa]

…accomodate dict-like parameter for different obs and model variable names. added unit tests for handle_sum that test both param types.

[github-actions]

Package	Line Rate	Branch Rate	Health
vxingest	31%	22%	❌
vxingest.builder_common	23%	11%	❌
vxingest.ctc_to_cb	11%	1%	❌
vxingest.grib2_to_cb	21%	6%	❌
vxingest.netcdf_to_cb	22%	13%	❌
vxingest.partial_sums_to_cb	22%	8%	❌
vxingest.prepbufr_to_cb	100%	100%	✔
vxingest.utilities	30%	35%	❌
Summary	21% (772 / 3642)	10% (94 / 918)	❌

[randytpierce]

Looks quite nice. I'll merge the integration test branch into this, make sure all the tests work, including the partial sums one and then merge to main.