Pytestless doctests 2

[pp-mo]

Replaces #177
Rebased to resolve conflicts,
and, since we moved the repo into SciTools, also replace the branch with one in my fork (pp-mo/ncdata)

##TODO
Issues inherited from #177

• explain that 'private' does not apply to docs files
• find better way, or document, the odd way we are doing globs
• tiny docs fix
• explain limited translation of args for doctests function calls, avoiding exec
• additional remaining
  • now complicated enough that it needs some tests
  • possible problems with less usual glob forms (in include/target specs)
  • support proper globs in exclude patterns ?
  • lack of global options config (? use a pyproject.toml section ?)

(and some more added since)

• output reports currently say like e.g.
  1 item had failures:
17 of 30 in lenient_metadata.rst
***Test Failed*** 17 failures and 9 skipped tests.
  • this should be clearer + especially include count of successes, so : "ok/bad/skip add up to total"
• ability to detect + skip internal sections that APPEAR like doctests, e.g. in ".. codeblock::"
  • .. but that is non-trival and would require deeper engagement with the doctests api, beyond 'testmod/testfile`