Remove notebook run checks

[ceball]

nbval can do notebook running checks, e.g. py.test --nbval-lax.

I'd probably prefer to remove nbsmoke's notebook running support entirely and have projects use nbval. But could consider replacing nbsmoke's notebook running code with a call to nbval's (i.e. keep --nbsmoke-run).

To do:

• get to know an author of nbval 😂
• check nbval supports cell timeout
• nbval does not support stopping on first error in the notebook?
• nbval does not record/show its own test coverage anywhere
• check nbval supports config via file (not just command line arguments) (it doesn't, but I think pytest's own stuff is ok for that, which is apparently what I thought years ago before I was persuaded to add special support ;) )
• check nbval supports skipping by regex/similar (hopefully better than what nbsmoke does; see complaints in Specifying skipped notebooks needs improvement #11) (it doesn't, but as above I feel pytest's own stuff is ok for that)
• actually do it :)

[ceball]

See also https://github.com/treebeardtech/nbmake