BUG: read_rsl now works with non-unform rays per sweep.

pyart.io.read_rsl can now read files correct which have different numbers
of rays per sweep.  Also assert statements were added to insure that
the various RSL structures being accessed in loop are not NULL, which
should prevent segmentation faults if errors do occur.
ENH: auto_read detects Lassen, use_rsl changed.

The read function will now auto detect Lassen files and read them.
The `use_rsl` parameter of the read function was also changed so that RSL is
always used to read files that are only supported by RSL. The parameter now
controls if RSL will read files supported natively and by RSL (ie Sigmet).

Py-ART can now read the example Gunn Point Lassen file, although the RHI scan
is read as a PPI.  Currenly RSL read all Lassen files as PPI scans ignoring the
scan_type parameter in the file.
Merge pull request #117 from jjhelmus/master

ENH: Support for reading Lassen files through RSL