Aacgmv2

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* Commented out debug statement in C code
* Updated environment variable warning to output to stderr instead of stdout
* Added templates for pull requests, issues, and a code of conduct

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* Updated C code and coefficients to version 2.5. Changes in python
code reflect changes in C code (includes going back to using environment
variables instead of strings for coefficient file locations)
* Added decorators to some of the test functions
* Specified AppVeyor Visual Studio version, since it was defaulting to 2010 and
that version doesn't work with python 3

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* Fixed bug in convert_mlt that caused all time inputs to occur
at 00:00:00 UT
* Fixed year of last two updates in changelog

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* Fix bug in installation that caused files to be placed in the wrong
directory
* Added DOI

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* Update to use AACGM-v2.4, which includes changes to the inverse MLT and
dipole tilt functions and some minor bug fixes
* Updated file structure
* Updated methods, retaining old methods in deprecated module
* Added testing for python 3.6
* Updated dependencies, removing support for python 3.3
* Tested on Mac OSX
* Updated comments to include units for input and output

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* Change method of calculating MLT, see documentation of convert_mlt for details