Add verification binary parametrization

Verification binaries are common (and certain) LISA sources. A catalog is available at https://gitlab.in2p3.fr/LISA/lisa-verification-binaries.

We should allow the user to use verification binary parametrization. There is a simple map between these parameters and the traditional parametrization used in GalacticBinary.

def compute_gb(lG, bG, P, m1, m2, DL, logger):
    """Convert galactic binary parameter into LDC conventions.

    Args:
        lG, bG: galactic coordinates
        P: period
        m1, m2: in solar mass
        DL: distance in pc

    Returns:
        bE, lE: ecliptic coordinates
        fGW: frequency
        Amp: amplitude
        dfodt: frequency derivative

    >>> bE, lE, fGW, Amp, dfodt = compute_gb(328.5, 53.1, 943.8, 0.8, 0.1, 1317)

    """
    logger.debug(f"lG = {lG}, bG = {bG}, P = {P}, m1 = {m1}, m2 = {m2}, DL = {DL}")
    xg = SkyCoord(lG, bG, unit='deg', frame='galactic')
    xe = xg.transform_to(BarycentricTrueEcliptic())
    bE = xe.lat.rad
    lE = xe.lon.rad
    M = (m1+m2) * MT_SUN
    eta = m1*m2 / (m1+m2)**2
    Mc = M * eta**(3./5.)
    DLs = DL*pc/c
    fGW = 2. / P
    Amp = 2 * (M**(5./3.) * eta/DLs) * (np.pi*fGW)**(2./3.)
    dfodt = (96./5.) * Mc**(5./3.) * np.pi**(8./3.) * fGW**(11./3.)
    return bE, lE, fGW, Amp, dfodt

Two solutions:

• Create a subclass of GalacticBinary that override the init method to take the verification binary parametrization as arguments (Galactic coordinates, period, masses, separation)
• Add a class method to GalacticBinary that takes the verification binary parametrization as arguments (Galactic coordinates, period, masses, separation) and initializes a GalacticBinary instance by converting these parameters