Add as_run parameter to ScmRun.process_over

[lewisjared]

Is your feature request related to a problem? Please describe.

The resulting output from process_over can often be cast to a ScmRun. A nice quality of life improvement would be performing this operation without an explicitly cast.

I would also recommend adding an op_cols parameter a la the ops functions. This would simplify when a required column such as variable is used.

This should allow the following:

df = run.process_over("variable", operation="sum")
df["variable"] = "Emissions|GHG"
run = scmdata.ScmRun(
  df
)

to be replaced with :

run = run.process_over("variable", operation="sum", ops_cols={"variable": "Emissions|GHG"}, as_run=True)

[znicholls]

as_run could allow metadata to be maintained too