Cosmological time integration

This is an absolute monster merge request.

It adds support for cosmological time integration. It has barely been tested for accuracy thus far. But I'd like to merge it in so we can move on with the accuracy tests. In principle, nothing is broken in the normal mode of operations and we should be able to run anything we currently do without any problems.

If this is confirmed and merged in, then I will update the examples we have to actually use the new module.

examples/CosmologicalBox/cosmo.yml

+# Define the system of units to use internally. 
+InternalUnitSystem:
+  UnitMass_in_cgs:     1.989e43      # 10^10 M_sun in grams
+  UnitLength_in_cgs:   3.085678e24   # Mpc in centimeters
+  UnitVelocity_in_cgs: 1e5           # km/s in centimeters per second
+  UnitCurrent_in_cgs:  1             # Amperes
+  UnitTemp_in_cgs:     1             # Kelvin
+
+#  UnitMass_in_cgs:     1   # Grams
+#  UnitLength_in_cgs:   1   # Centimeters
+#  UnitVelocity_in_cgs: 1   # Centimeters per second
+#  UnitCurrent_in_cgs:  1   # Amperes
+#  UnitTemp_in_cgs:     1   # Kelvin
+
+# Parameters governing the time integration
+TimeIntegration:
+  time_begin: 0.    # The starting time of the simulation (in internal units).
+  time_end:   1.    # The end time of the simulation (in internal units).
+  dt_min:     1e-6  # The minimal time-step size of the simulation (in internal units).
+  dt_max:     1e-2  # The maximal time-step size of the simulation (in internal units).
+
+# Parameters governing the snapshots
+Snapshots:
+  basename:            uniformBox # Common part of the name of output files
+  time_first:          0.         # Time of the first output (in internal units)
+  delta_time:          0.01       # Time difference between consecutive outputs (in internal units)
+
+# Parameters governing the conserved quantities statistics
+Statistics:
+  delta_time:          1e-2 # Time between statistics output
+
+# Parameters for the hydrodynamics scheme
+SPH:
+  resolution_eta:        1.2348   # Target smoothing length in units of the mean inter-particle separation (1.2348 == 48Ngbs with the cubic spline kernel).
+  CFL_condition:         0.1      # Courant-Friedrich-Levy condition for time integration.
+  
+# Parameters related to the initial conditions
+InitialConditions:
+  file_name:  ./uniformBox.hdf5     # The file to read
+
+Cosmology:
+  Omega_m:        0.307
+  Omega_lambda:   0.693
+  Omega_b:        0.0455
+  h:              0.6777
+  a_begin:        0.0078125
+  a_end:          1.0
+  
+