v0.3.0: introduced proton particles distribution

v0.3.0 introduces proton particles distributions.

In the previous versions of the package, only an electron distribution was available, the latter had to be defined through a dictionary

# set the spectrum normalisation (total energy in electrons in this case)
spectrum_norm = 1e48 * u.Unit("erg")
# define the spectral function parametrisation through a dictionary
spectrum_dict = {
    "type": "PowerLaw",
    "parameters": {"p": 2.8, "gamma_min": 1e2, "gamma_max": 1e7},
}

# set the remaining quantities defining the blob
R_b = 1e16 * u.cm
B = 1 * u.G
z = Distance(1e27, unit=u.cm).z
delta_D = 10
Gamma = 10

blob = Blob(R_b, z, delta_D, Gamma, B, spectrum_norm, spectrum_dict)

This was not very handy: I have simplified - and broke - the previous blob API.
From version 0.3.0 this is how electron distribution are defined and passed to the blob

# electron distribution
n_e = BrokenPowerLaw(
    k=1e-8 * u.Unit("cm-3"),
    p1=1.9,
    p2=2.6,
    gamma_b=1e4,
    gamma_min=10,
    gamma_max=1e6,
    mass=m_e,
)

# set the quantities defining the blob
R_b = 1e16 * u.cm
z = Distance(1e27, unit=u.cm).z
delta_D = 10
Gamma = 10
B = 1 * u.G

blob = Blob(R_b, z, delta_D, Gamma, B, n_e=n_e)

The new way of defining particle distributions also allows to define proton distributions

n_p = PowerLaw(k=0.1 * u.Unit("cm-3"), p=2.3, gamma_min=10, gamma_max=1e6, mass=m_p)
blob = Blob(R_b, z, delta_D, Gamma, B, n_e=n_e, n_p=n_p)

The only difference between particle distributions is the mass argument.
This will eventually allow to describe other particle distributions.

To achieve this:

• ElectronDistribution has been changed to ParticleDistribution. This is the base class from which all the different distributions (PowerLaw, BrokenPowerLaw etc..) inherit;

• @dimaniad6 additionally implemented an InterpolatedDistribution to read an arbitrary input particle distribution (two arrays: one for of Lorentz factors and the other for densities);

• the Blob class has been improved:

  • the different initialisation method available under the spectrum_norm_type argument of the Blob are now implemented in the base ParticleDistribution class. I.e. one can initialise from_total_density, from_total_energy_density, from_density_at_gamma_1, from_total_energy;
  • several quantities are now provided as properties and are evaluated on the fly from the base parameters ( $R_b$, $z$, $\delta_D$, $\Gamma$, $B$);

• the other classes remain largely unchanged. They have just been modified internally to deal with the new definition of particle distributions;

• note that the prefactor of the particle distribution has been renamed from k_e to k, as we are not describing exclusively electrons distributions now. This means that the corresponding parameter in the fit classes is log10_k instead of log10_k_e.