docs(mf6ivar): readme.md updated to include valid keyword (#60)

* refactor(pre-commit.py): use OrderdedDict to load and write code.json

Updated the code.json writer to use an ordered dictionary so that the order is always the same.  This will minimize the number of changes that appear in the changelog due to ordering changes that do not change content.

* fix(GwfGwfExchangeModule): specific discharge not updated correctly for LGR

The sign for the face normal needed to be flipped for model 2 so that specific discharge would be calculated correctly.  Added a test to make sure that the velocities are correct.  Addresses #51.

* fix(GwfGwfExchangeModule): specific discharge not updated correctly for LGR

Updated the release notes and added some docstrings to the autotest.

* docs(mf6ivar): readme.md needed a description of the valid keyword

doc/mf6io/mf6ivar/readme.md

@@ -11,6 +11,7 @@ A MODFLOW 6 variable, and its place within a block and on a line is defined usin
 * block -- this is the name of the block that contains the variable.  Required.
 * name -- this is the name of the variable.  Required.
 * type -- this is the type of variable.  Valid values are: keyword, string, integer, double precision, recarray, record, recordrepeating, keystring.  Required.
+* valid -- list of valid values or keywords
 * shape -- this is the size of the array.  Only required for arrays.  Optional.
 * tagged -- if set to false, then a keyword is not required prior to value itself.  tagged is set to true if not specified.  Optional.
 * in_record -- if true, then this means that the variable is part of a record, and so it should not be listed on its own line.  in_record is False if not specified.  Optional.