Added initial code for local index definition of grib files.

[Anu-Ra-g]

This is thed demo for an initial version of the local grib index file defintions. The code still needs more update. Below is the output created in a .idx file

1:0:d=2023092800:V:6 hour fcst
2:955132:d=2023092800:ABSV:6 hour fcst
3:1898414:d=2023092800:CLWMR:6 hour fcst
4:2243950:d=2023092800:ICMR:6 hour fcst
5:2519285:d=2023092800:RWMR:6 hour fcst
6:2800049:d=2023092800:SNMR:6 hour fcst
7:2922175:d=2023092800:GRLE:6 hour fcst
8:2995158:d=2023092800:O3MR:6 hour fcst
...

[martindurant]

I have no idea of course if the code is right (and there are no tests yet); but what is the use-case for this? If we scan the file, we already know the full information as encoded in the kerchunk references, so why would we want to make an .idx too?

[martindurant]

Pass a filename or maybe open file?
Dumping a load of files into the local directory doesn't seem like a great idea.

[Anu-Ra-g]

I thought local definitions of .idx for the grib files might be a good idea. This was just for demo purpose.

[martindurant]

This was just for demo purpose.

That's fine, but please update the PR description or title to say so.