set nw.start default to "first"

[Martina Morris]

currently "NULL", and if not passed it generates erroneous results with no error, see https://github.com/statnet/Workshops-private/issues/24

"first" will probably cover 99% of actual use cases.

[Pavel N. Krivitsky]

I am not sure. For example, a natural use case is to continue the network series into the future, in which case it would be the last.

[Martina Morris]

Ok, maybe not 99% of the use cases, but I still think first is a more natural default.

[Pavel N. Krivitsky]

For now, I would rather the user specify.