Inclusion of BATSE systematic error

[eburnsastro]

In gdt-fermi gdt.missions.fermi.gbm.localization contains a few different GBM systematic errors (ground position, hitl, roboBA, etc). It would be beneficial to have a similar file handling the BATSE systematic options. A basic one is below, but I believe the reference contains more complex representations which consider spacecraft pointing, etc

def BATSE_Basic_Model():
    """The localization systematic model for the Ground-Automated localization:
    A mixture of a 1.85 deg Gaussian (78% weight) and a 5.1 deg Gaussian.

    Note: a more detailed systematic is available, dependent on data used
    
    References:
        https://iopscience.iop.org/article/10.1086/313221/pdf
    """
    sigma1 = np.deg2rad(1.85)
    sigma2 = np.deg2rad(5.1)
    frac1 = 0.78
    return ([sigma1, sigma2], [frac1])