Polar2grid

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* Start using Pytroll SatPy library for various features
* Phasing out of legacy <reader><writer>.bash scripts and replacing with polar2grid.sh <reader> <writer> implementation.
* Add ability to output float geotiffs
* Add ability to store geotiff tiles instead of strips
* Fix fornav bug for non-float inputs (unused in most cases)
* Add `add_colormap.sh` script for adding color tables to geotiffs
* Add `add_coastlines.sh` script for adding borders, coastlines, rivers, etc to geotiffs
* Add basic NUCAPS reader (via SatPy)
* Add VIIRS L1B reader (via SatPy)
* Add AMSR2 L1B reader (via SatPy)
* Add MIRS reader
* Change default `fornav-d` flag in most glue scripts to `1`
* Adjust true/false color scaling to be more continuous (similar result)
* Add AWIPS Puerto Rico Grid (210)
* Add `polar_alaska` dynamic grid
* New version of ShellB3 for C/python dependencies
* Fix geotiff geotransform to fix "off by half-pixel" bug.

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* Fixed small bug in ll2cr where NaNs in navigation would cause a dynamic grid to never "fit"

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* Rewrite of entire internal structure and behavior of polar2grid (Frontends, Backends, Remapping)
* Most frontends (VIIRS, MODIS, etc) are filename independent and try to determine type of file by internal structure
* Frontends now do operations based on what "products" are requested and return a "scene" object
* The `polar2grid.core.meta` module is added to provide structure to intermediate steps (Frontend -> Remap -> Backend) with the classes it offers
* Backends now operate on a gridded scene as a whole (with option for operating on one product at a time for some backends)
* A compositor role was added to provide a more flexible method of creating true/false color and other composited images
* ll2cr rewritten in python and cython (C-like python) to be faster and more accurate
* Grid determination has been essentially removed since "data fits in grid" decisions don't make sense unless you are in projection/grid space (ll2cr serves this purpose now)
* fornav has been rewritten to be accessed directly from python. The ms2gt version of fornav is no longer used.
* The ms2gt version of fornav was also modified to be faster and is still destributed with the software bundle (for this release only).
* GPD grids and support for them has been removed. PROJ.4 is more flexible, more widely used, and can actually support the AWIPS grids better.
* Python setup.py files updated to better meet common practice of other python projects (READMEs, classifiers, etc)
* Major changes to rescaling so that it can be specified independent of output data type and "increment_by_one"
* Removed AWIPS I support due to National Weather Service using AWIPS II from now on
* Added basic ACSPO and MIRS frontends
* Added HDF5 backend

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* Fixed VIIRS CREFL C code and added custom version to repository (viirs_crefl)
* Added ability to use 'deg' units on grid origin definitions
* Latlong grids use degrees instead of radians (affects grid configurations and intermediate values in code)
* Added `wgs84_fit_250` grid
* AWIPS grids "fixed" to actually align properly in AWIPS. The grid specification says ellipsoid earth, but my results say spherical.

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* Non-TC geolocation used as backup option for VIIRS Frontend
* Fixed major bug when creating true colors (Issue 81). If the high resolution data resolved to a different dynamic grid than the low resolution data then a true color could not be made.

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* Fixed frontends handling of symbolic links for files