Correct several errors in the complex packing decoder

[noritada]

This PR makes following corrections of errors in the complex packing decoder:

• Correctly use the first values of undifferenced scaled data values defined at the beginning of Section 7
• Add support for zero width groups
• Correct coding errors in offset bit calculation in handling non-zero width groups

The first two of the three correct errors caused by lack of the complex packing specification.

Closes #29.