Add support for more recent netCDF data types.

This is a partial resolution of issue 54, but there is still no support for
strings of arbitrary length.

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branch : release-0.7
Release 0.7.3.

--HG--
branch : release-0.7
Added tag 0.7.3 for changeset 7bafc0b811e2

--HG--
branch : release-0.7
Fix the last "fix" to netcdf.

When saving a version 3 file, a lot of these new data types are not available,
so need to have 2 variants of the data type lookup table.

Also, forgot a couple of lookup tables related to saving/loading variables.

--HG--
branch : release-0.7
Added tag 0.7.3a for changeset 32bac361a52c

--HG--
branch : release-0.7