ENH: Refactor and improvements of FourDD code.

* Existing FourDD code base was refactor to improve readability and
  maintainablilty.
* Compile time parameters were exposed and can be set from Python during run
  time.
* Added support for dealiasing radars where the number of ray varies between
  sweeps.
* The functionality of the FourD dealiasing algorithm was not modified, no
  bugs where fixed, etc.  This version should give identical results as older
  version of the code.  THIS WILL BE THE LAST COMMIT WHERE THIS IS TRUE.
* A full revision history of changes made during this refactor can be found
  in the fourdd_fixes branch of GitHub user jjhelmus.
BUG: FourDD filtering based on reflectivity

When filt=1 is used during dealiasing gates where the reflectivities are
below lowdbz or above highdbz are now masked in the velocity field as well as
gates where the reflectivity is missing if rm_missing is set to 1.
BUG: wrong ineqality in FourDD filter
WIP: conda recipe
ENH: Added additional RSL Volume numbers
BUG: RSL fields not included in field mapping not dropped.

Unassigned RSL fields are not mapped to a field of name None,
rather they are skipped.
ENH: New CI Setup

* Conda recipe tested.
* Documentation is build whenever commit lands on ARM-DOE/master.
BUG: errors cause early end in build_docs.sh
Merge branch 'master' into enh_ci

Conflicts:
	pyart/correct/_fourdd_interface.c
	pyart/correct/_fourdd_interface.pyx
Merge branch 'enh_ci'
Merge branch 'master' of https://github.com/jjhelmus/pyart