Introduces Initial values block (cf. #368 for the discussion)

[DimitriPlotnikov]

ODEs are now modelled through the equations block and initial_values block. Shapes can be stated as an ODE (or a system of) or a functions of t.

The example syntax for 2 alpha shapes is now:

 initial_values:
    g_in nS = 0nS
    g_in' nS/ms = nS * e / tau_syn_in
    g_ex nS = 0nS
    g_ex' nS/ms = nS * e / tau_syn_ex
  end

  equations:
    shape g_in'' = (-1)/(tau_syn_in)**(2)*g_in+(-2)/tau_syn_in*g_in'
    shape g_ex'' = (-1)/(tau_syn_ex)**(2)*g_ex+(-2)/tau_syn_ex*g_ex'
  end