Bring back the EXPORT for PyImath.

[lamiller0]

Address Issue #395 where packages that need to link against PyImath can no longer find it.

[lgritz]

From discussion at the last TSC meeting, we decided that what was really wrong originally that led to the export being removed is that somehow we were installing the module in two places, one of them (incorrectly) in the lib area, instead of strictly in the python site-packages area. THAT is what should be fixed, and if so, then the EXPORT is correct.

Please correct me if my recollection or explanation is wrong, @kdt3rd @meshula @cary-ilm.

We were a little unclear about exactly who should do that fix, and whether we should just go ahead and merge this PR on its own (put the EXPORT back) and separately (but very soon) fix the installation issue, or if we should make a point to do both fixes as one PR.

[meshula]

@lgritz Yes, you're recalling right. We discussed that we should be pointing into site-packages. We didn't solicit a volunteer though.

[lgritz]

Alternate proposal here: #400