swetest: new formula

The [SWISS EPHEMERIS](http://www.astro.com/swisseph/) is the high precision ephemeris developed by Astrodienst, largely based upon the DExxx ephemerides from NASA's JPL 

From Homebrew/homebrew#29636
The best would be to run swetest on sample data, or to run a built-in test suite if any. `mafft` puts together very simple test data, `mumps` runs a few built-in tests. Grep through formulae for `make test` for install-time tests.