Note CdfReader class as starting point for low-level reads
Fix for use with non-sparse variables that have no records.
Fix TIME_TT2000 data type bug.

Conversions were way out - decimal point error.
They're still not correct (leap seconds), but the discrepancy is now
of the order of a minute.
Fix some problems with tests
Special case for TIME_TT2000 default pad value.

Sec 2.3.20 of the CDF users guide says that Long.MIN_VALUE+1 is to
be interpreted with a special case value of 0AD.
Implement default pad values correctly.

Previously sparse records caused an error if the pad value was not
explicitly supplied.
Add new test file.

This one, test.cdf, is from cdf34_1-dist/cdfjava/examples/test.cdf.
It contains one of each column type.  Thanks to Michael Liu (NASA)
for drawing it to my attention.
Fix EPOCH16 bug
Add EPOCH16 formatting
Run some tests against data/test.cdf file as well.

Tests not complete at this commit - in particular TT_TIME2000 is not
working.