Tool to convert temperature to potential temperature

- Uses EOS80 equation of state (known to be obsolete but it works!).
- Uses depth in meters as approximation for pressure in dbars.

To do:
- Switch to TEOS-10
- Use more accurate pressure calculation
Added Makefile rule to generate potential temperatures
Added tool for joining multiple files into one file

- I couldn't figure out how to use nco to do this when
  there is no record dimension to concatenate. It was quicker
  to write the tool than figure out how to do the conventional
  way...
Added quick and dirty comparison tool

- Calculates layer-by-layer stats and highlights outlier data.
Rules to create final netcdf files
Tried alternative pressure approximations

- Added alternative approximations for pressure in potential
  temperature calculation.
- Alternatives are commented out. Made differences ~ 2e-3 degrees
  Celsius.
Added mean difference statistic to compare2netcdf.py
Separated rules for generation and comparison of final output
Extended meta information in netcdf attributes

- Addded a lot of attributes based on WOA09 product.