Chooses eval engine automatically depending on whether numexpr is installed

[robintw]

Fixes #12008

[jreback]

can you add some tests for this (e.g. the example from the issue).
The 2.7_LOCALE build (the 2nd one down on the travis builds), does NOT have numexpr installed (its the only one). So if you mark this tests as slow it will get run there (and should pass with the python parser) and on the other builds (don't mark as slow) with the numexpr engine.

So the tests should assert the state of _NUMEXPR_INSTALLED (IOW whichever you are actually testsing), then run the query. I don't think there is an easy way to figure out which engine is actually used (we may have to do some gymnastics here to assert that)

[jreback]

this needs a whatsnew note, put in API changes (even though it is more of a bug-fix)

[robintw]

Which whatsnew file should it be put in? 0.18.1?

Also, I'm a little confused about the tests: surely the test is either marked as slow or not marked as slow? I assume the tests should go in test_query_eval.py, but having looked at the tests in there I would have thought they would have caught this bug previously...

I'm happy to have a go at writing a test, with a bit of help, but I may not be the best person to do this - and someone else may be able to do it in a tiny fraction of the time.

[jreback]

0.18.1

you need 2 tests the same 1 slow and 1 not-slow (don't mark it)
they will be run in separate builds

[jreback]

replaced by 504ad46

but #12867 is still relevant (e.g. we may want to show a PerformanceWarning when numexpr is not installed