[config] Set the exponent property as mandatory when density is exponential

[ftsamis]

When model.structure.density is of type exponential, the exponent property was not specified in the config definition neither as required nor as optional, making the validator allowing the following as valid input (note that exponent is missing).

density:
  type : exponential
  time_0: 2. day
  rho_0: 6.e-10 g/cm^3
  v_0: 3000.  km/s

With this PR I'm adding the exponent property as a mandatory one for the exponential density type.

I mainly open a new PR for that because I want to avoid semantic differences between tardis_config_definition.yml and the new json schema at #549