Implement UGRID default start_index

[pp-mo]

✨ Feature Request

In the UGRID conventions, the 'start_index' property is never actually required,
e.g. in this section

For the indexing one may use either 0- or 1-based indexing; the convention used should be specified using a start_index attribute to the index variable (i.e. Mesh2_face_nodes in the example below). Consistent with the CF-conventions compression option, the connectivity indices are 0-based by default. See this section on zero or one-based indexing for more details.

But in our current code, we have assumed it is always there

Notable specifically for connectivities,
might possibly apply elsewhere, but at present I think not : probably just in this one place ?

[trexfeathers]

This should I think be fine to implement - the Connectivity class always has a start_index attribute, but defaults to =0 in line with the UGRID conventions.

[pp-mo]

fine to implement

Absolutely, logically it is all fine, we must just stop assuming that a file variable always has a start_index attribute.

So, I think this PR should be all that's needed

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