Change initialization of NDData arithmetic result to include keywords in constructor
Initialize result with keywords in constructor call
Add note to docstring indicating that flags are set to None

This is not a change in behavior in convert_unit_to, just indicating in the doctoring what is being done.
Add CHANGES.rst entry

Closes #2300
Add tests for #2300
Merge pull request #2301 from mwcraig/nddata-init-hack

Make initialization of results in arithmetic and unit conversion to allow more flexibility in subclasses