MAINT: write_cfradial write altitude_agl variable.
MAINT: Refactor of instrument param handeling.
ENH: write_cfradial write radar_calibration variables.
ENH: write_cfradial writes scan_rate and antenna_transition variables.
ENH: CfRadial target_scan_rate variable read and written.
ENH: optional global variables written in CfRadial files.
ENH: rays_are_index and ray_angle_res support in cfradial.py
BUG: Extra scaling and masking of data read from CfRadial files.

* scale_factor, add_offset, missing_value, and _FillValue attributes are
  handled by the python-netCDF library for masking and scaling of the data,
  these should not be included in the metadata dictionary.
MAINT: _FillValue and missing_value included in metadata

* Reverted portion of last commit, _FillValue and/or missing_value
  should be included in the metadata.
BUG: Empty radar_calibration attribute bug fixed.
BUG: Fixed issue in reading CfRadial files with varying gates per ray.

closed #207
Merge pull request #210 from jjhelmus/cfradial_fixes

Numerous bug fixes and enhancements to the cfradial.py module