add DataType.toString
Add -html option for CdfDump.

The point of this is to make it easy to chase record pointers by
presenting them as hyperlinks.  Easy to do.
Fix a bug with pad value reading.

Pad values are only 1 item long; previously it was attempting to read
rawItemCount items.
Add a unit test
Add a couple of example CDF files for testing.

These files are from the CDF 3.4 distribution (cdf34_1-dist/samples).
Tweak test case details
Add another test case.

This one tests the content of a couple of example files with known content.