Semi working writer.
Most tests now passing.
Merge pull request #10 from kiyo-masui/sdfits

Sdfits
Fits code fully ported.
Doc-strings for SpecWriter.
Merge pull request #11 from kiyo-masui/sdfits

Sdfits
file_utils unittests and doc.
Merge pull request #12 from kiyo-masui/utils

file_utils unittests and doc.