Add VLArrayCol / support variable length arrays in IsDescription

[maxnoe]

Support for storing variable length arrays is currently missing in pytables.

From the hdf5 documentation, this should in principle be possible.

What would be needed to imlement a VLArrayCol?

[avalentino]

Dear @maxnoe thank you for your interest in PyTables.
It seems that there is a lot of interest for supporting VL* objects in PyTables.

Personally I have not so much time to dedicate to PyTables in this period so I can't promise anything.
I suspect that the same holds for other PT developers.

Anyway if you want to submit a PR to add this feature (including a minimal testing) I will be happy to review and merge.

[HealthyPear]

@avalentino I am also interested in supporting loading from HDF5 a table like this

A	B
2	[1, 2]
4	[1, 2, 3, 4]

and I think awkward-array could be used to implement this

[avalentino]

Dear @HealthyPear
I don't know awkward-array.
I'm not sure to understand if it is possible to map an awkward-array into an HDF5 type in a reliable way.
Do you have already an idea about how to do it?

[HealthyPear]

Unfortunately not an expert...

I was thinking of starting this as a very complex exercise from scratch and create a new plugin for awkward for HDF5 (much like awkward-pandas), but there is also the alternative to add what's missing here (less code rewriting I guess...)

To my understanding at least one ingredient would be this
https://awkward-array.org/doc/main/user-guide/how-to-convert-buffers.html