New cork options "-first" and "-second" to produce A U (A^B) and B U (A^B)

[fangq]

two changes were found necessary in order to use cork in my project:

1. I had to bump the output precision in the off file higher to avoid topological errors due to floating-point errors when using the output off files for tetrahedral mesh generation (using tetgen). see commit 971cb94
2. I added boolFirst() method to generate surface A U (A^B), and added -first option in the cork binary. see commit 3faf80d. -first outputs surface input#1 cut by surface input#2.
3. I also added -second to create B U (A^B), but may have some issues.

I also included the modified cork binary in my mesh generator project iso2mesh to replace the outdated libgts boolean operator.

https://github.com/fangq/iso2mesh/commit/2acea45de866f94682a9495e0d8c0335c6ee7d71