rewrite reac_density() for the three cases

[riberaborrell]

No description provided.

[riberaborrell]

Split the method reac_density() in three methods:

• reac_density(): not normalized reactive density
• reac_norm_factor(): normalization factor
• norm_reac_density(): normalized reactive density

[LuzieH]

• for the norm_reac_density() method: ensure that the density is only computed for the times when the normalization factor is >0 (this can also be the case for times 0<n<N-1

• further, ensure that the norm_reac_density method doesn't compute reac_dens and _reac_norm_factor again, if they have already been computed

• small network example: there is a problem with plotting the finite time reactive density for times n=0 and n=N-1