Handle missing netCDF4 dependency

[Mirochill]

Summary

• Keep a module-level netCDF4 = None marker when the optional NetCDF dependency is not installed.
• Make cnv2nc() fail with an actionable ImportError that points to pip install 'seabird[CDF]' or pip install netCDF4 instead of continuing to a NameError.
• Add a regression test for the missing-netCDF4 path.

Validation

• git diff --check HEAD~1..HEAD
• Static search for the new missing-dependency guard and regression test
• Tests not run locally.

Fixes #77