Allow writing of LAS 3 sections (i.e. non standard)

[kinverarity1]

Ref email enquiry.

Need to modify writer.py to allow the addition of sections which are not contained in the LAS 2 specs e.g. Tops_Parameter, Tops_Definition, Tops_Data

[ghost]

This would be a useful enhancement!

[kinverarity1]

This could be done nicely by sublcassing lasio.las_items.SectionItems for the different types of sections, and implementing a write() method. Then the same method could be also be used for __str__().

And las.write() would essentially iterate over the sections and call write() for each.

The implementation would need to be LAS-version-aware, but perhaps the LAS version only gets used on las.write() itself, and by default i.e. str(las.sections["~Version"]) would render as LAS 2. Not sure about that bit. Would be easy to change.

Relevant issues are: #182, #266, perhaps #268,