implement dirint

[wholmgren]

This PR implements the dirint function based on the MATLAB pvl_dirint.m file. Closes #4.

I've not yet tested the implementation very thoroughly, so let's hold off on merging until I write a few tests.

As suggested in #42, I would prefer to put this function and the disc function in irradiance.py instead of clearsky.py.

[wholmgren]

I think I've tested the dirint function well enough for now. I pulled 4 days of GHI and DNI measurements here in Tucson, AZ, ran them through disc and dirint, and compared the results to DNI measurements. The 'DNI clear' line comes from clearsky.ineichen.

There are some more specific functional and physical tests in the test suite.

Note the small change in the disc algorithm to make this work (0 instead of nan if calculation gives less than 0).

Again, happy to make improvements, but if nobody cares, I suggest a merge so that other people can discover my mistakes.