ENH: Faster reading of sigmet files and support for missing rays

* Sigmet files are read as speeds similar to RSL using the Cython SigmetFile
  object.
* Sigmet files which contain rays which were not collected are returned as
  proper radar object with the missing rays excluded from the data.

For comparison before this refactor reading a sample XSAPR PPI using the SigmetFile object took 7.2 seconds, now reading the same files takes 1.0 seconds.  Loading the data into a RslFile object takes 1.1 seconds.