LABEL_SELFREF enhancements

[yakra]

• Avoid false positives at U-turn ramps.
  I'll have another pull request along in HighwayData shortly to remove the newly-unmatched FP entries.
• Flag letter-suffixed routes after a slash.
  Closes LABEL_SELFREF on letter-suffixed routes #471.
• Optimize for speed.
  Closes LABEL_SELFREF efficiency #173.
  Python-flavored datacheck time is down about 25%. That's for all datachecks combined. Wow. LABEL_SELFREF was a big chunk of that total!

[jteresco]

This will be used starting tonight, thanks!

[yakra]

Just took a look back at the datacheck page, specifically at CHL.
Most of what's there looks dubious:

chl.r005;R5_U1013S;;;LABEL_SELFREF;
chl.r005;R5_U1026S;;;LABEL_SELFREF;
chl.r005;R5_U1030S;;;LABEL_SELFREF;
chl.r005;R5_U1059S;;;LABEL_SELFREF;
chl.r005;R5_U294N;;;LABEL_SELFREF;
chl.r005;R5_U301N;;;LABEL_SELFREF;
chl.r005;R5_U311N;;;LABEL_SELFREF;
chl.r005;R5_U320N;;;LABEL_SELFREF;
chl.r005;R5_U328N;;;LABEL_SELFREF;

These are obviously right out, not following U-turn nomenclature.
I'll have to go back and make the holes in the net a bit smaller.

Then there's
chl.r005;R5_U559;;;LABEL_SELFREF;
which meets the standard except that there's no R5_U1 thru R5_U558.
These can be flagged, though it gets a bit more complicated. Most likely will involve a new datacheck code.

[yakra]

• What happens with labels ending in an underscore?
  IndexError. Putting in a try/except condition for these rare cases.