Documentation loic

Update a little bit the structure and add cooling.

I have created a new directory called NewOption where I have written the most important step to do when creating a new option. I am using some links between the files to make it easily accessible and avoid useless copy. Feel free to use it in the Hydro.

By the way, we have a documentation milestone now.

doc/RTD/source/Cooling/index.rst

+.. Equation of State
+   Loic Hausammann, 7th April 2018
+
+.. _cooling:
+
+Cooling
+=======
+
+Currently, we have 5 different cooling (EAGLE, Grackle, const-lambda, const-du
+and none).  Three of them are easily solved analytically (const-lambda,
+const-du and none) while the two last requires complex chemical networks.
+
+
+Equations
+---------
+
+The first table compares the different analytical cooling while the next ones
+are specific to a given cooling.  The quantities are the internal energy (\\( u
+\\)), the density \\( rho \\), the element mass fraction (\\( X_i \\)), the
+cooling function (\\(\\Lambda\\), the proton mass (\\( m_H \\)) and the time
+step condition (\\( t\_\\text{step}\\)).  If not specified otherwise, all
+cooling contains a temperature floor avoiding negative temperature.
+
+.. csv-table:: Analytical Cooling
+   :header: "Variable", "Const-Lambda", "Const-du", "None"
+
+   "\\( \\frac{ \\mathrm{d}u }{ \\mathrm{d}t } \\)", "\\( -\\Lambda \\frac{\\rho^2 X_H^2}{\\rho m_H^2} \\)", "const", "0"
+   "\\( \\Delta t\_\\text{max} \\)", "\\( t\_\\text{step} \\frac{u}{\\left|\\frac{ \\mathrm{d}u }{ \\mathrm{d}t }\\right|} \\)", "\\( t\_\\text{step} \\frac{u}{\\ \\left| \\frac{ \\mathrm{d}u }{ \\mathrm{d}t }\\right|} \\)", "None"
+
+
+Grackle
+~~~~~~~
+   
+Grackle is a chemistry and cooling library presented in B. Smith et al. 2016
+(do not forget to cite if used).  Four different modes are available:
+equilibrium, 6 species network (H, H\\( ^+ \\), e\\( ^- \\), He, He\\( ^+ \\)
+and He\\( ^{++} \\)), 9 species network (adds H\\(^-\\), H\\(_2\\) and
+H\\(_2^+\\)) and 12 species (adds D, D\\(^+\\) and HD).  Following the same
+order, the swift cooling options are ``grackle``, ``grackle1``, ``grackle2``
+and ``grackle3`` (the numbers correspond to the value of
+``primordial_chemistry`` in Grackle).  It also includes some self-shielding
+methods and UV background.  In order to use the Grackle cooling, you will need
+to provide an HDF5 table computed by Cloudy.
+
+When starting a simulation without providing the different fractions, the code
+supposes an equilibrium and computes the fractions automatically.
+
+Eagle
+~~~~~
+
+TODO
+
+How to Implement a New Cooling
+------------------------------
+
+The developper should provide at least one function for:
+ * writing the cooling name in HDF5
+ * cooling a particle
+ * the maximal time step possible
+ * initializing a particle
+ * computing the total energy radiated by a particle
+ * initializing the cooling parameters
+ * printing the cooling type
+
+For implementation details, see ``src/cooling/none/cooling.h``
+
+See :ref:`new_option` for the full list of changes required.