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.