Switch to key = value format for _prob_params

[maxpkatz]

The _prob_params script (and perhaps later on _cpp_parameters and the extern variables) should be switched to a key = value format so that we can more easily add fields in the future. For example we could do Python's ConfigParser format so that

p_ambient              real                 1.e-5_rt                     y                                                                                                                                        

becomes (perhaps)

[p_ambient]
type = real
default = 1.e-5_rt
namelist = True

[eliodilar]

i can work on it to begin as developper if it's possible?