* Implement a module to calculate seawater density using simple nonlinear EOS.
* Implement some routines to provide the conversion between potential temperature and in situ temperature
  using the formulation in which the extent of approximation is same as it corresponding each EOS.
* Add some graphs of density, Cp and difference between temperature and potential temperature.
* Write the descriptions for the feature of graphs.

commit 14da746942cbdca37f3630464e75b87ff902a23b
Author: Yuta Kawai <ykawai@gfd-dennou.org>
Date:   Thu Mar 6 19:24:09 2014 +0900

    * Modify some documents.

commit 62f71fd604ca1048b2df4a958ab089b02f098860
Author: Kawai Yuta <ykawai@Kawai-no-MacBook-Air.local>
Date:   Thu Mar 6 16:57:45 2014 +0900

    * Add some descriptions for primary features of density, Cp and Theta-T graphs.

commit 8f6d82ee1fc593e3e446854de47e09ea95c2984b
Author: Yuta Kawai <ykawai@gfd-dennou.org>
Date:   Mon Mar 3 01:52:11 2014 +0900

    * Add some graphs and the description for specific heat and the difference between temperature and potential temperature
      calculated by three diffrent formulations.

commit 5af90f2d5ae66be120345ddaf35295ab435ac5e1
Author: Yuta Kawai <ykawai@gfd-dennou.org>
Date:   Mon Feb 24 21:38:04 2014 +0900

    * Add some graphs of density calculated by Linear/SimpleNonLinear EOS and
      the description about their error relative to JM95 EOS.

commit c247c948620a73bc71edf9da1e1e6fd06674dc13
Author: Yuta Kawai <ykawai@gfd-dennou.org>
Date:   Sun Feb 23 18:25:46 2014 +0900

    * Move the declaration of id and name tag for each EOS into the corresponding module from GoverEqSet_mod.
    * Add a module to calculate th density using  a simple (idealized) nonlinear seawater EOS.