Convert connectivity variable data from file's start_index convention to caller-preferred start_index.

[arthurvd]

Is your feature request related to a problem? Please describe.
The UGRID topology tables (e.g. face_nodes) contain the integer index references as they occur in the input file read, with the start_index convention used in that same file. A calling application might want to use different 0/1-based indexing, so it would be nice to be able to pass a caller's start_index to request that when calling mesh2d.get*-like functions.

Describe the solution you'd like
I don't know yet which API function(s) are the best to receive such an extra input argument.
But, the given start_index must be used to convert the index values read from file, as it was also done in the predecessor ("io_netcdf").

It is important to respect any _FillValue present in the file: missing values must not be shifted by +/-1.

Describe alternatives you've considered
Let the client do this itself. A bit less user-friendly.

Additional context
See issue #3, which must be finished before this issue can be picked up.