Link LAS 3 "data set" sections together

[kinverarity1]

Is your feature request related to a problem? Please describe.
LAS 3 files can have linked parameter, curve ("definition") and data sections. These are currently independent sections in lasio. They should be linked.

See pages 4 and 5, and 17ff of the spec for more details. https://github.com/kinverarity1/lasio/blob/main/standards/LAS_3_File_Structure.pdf

Describe the solution you'd like
Full details of the API can be worked out, but as a starting point I suggest something like (pseudo-code):

>>> las.sections.keys()
['~Tops_Data', '~Tops_Definition', '~Tops_Parameter', ...]
>>> las.datasets.keys()
['Tops', ...]
>>> las.datasets['Tops']
{
'data': ?, 'defn': <type lasio.SectionItems>, 'param': <type lasio.SectionItems>,
'_sections': [...] # filtered version of las.sections
}

Describe alternatives you've considered
Open to other ideas, please discuss