Add convenient Tinygraph construction functions

[daniel-j-h]

At the moment the Python impl. Tinygraph can only be constructed from the two arrays: an offsets array and a targets array.

We should - for convenience - provide construction e.g. from an iterable of (from, to) tuples or construction from two parallel iterables of [from0, from1, ..] and [to0, to1, ..] shape.

Note: we might want to look into networkx and scipy's sparse matrices and how to convert from/to them. Maybe split off into new ticket.