Cannot create create fault skin for classic fault skin

[haizadtarik]

Current implementation of the function _load_skin only enable new fault skin to be loaded since old fault skin only have 1 header.
The function need to be modified as follows:

def _load_skin(filename: str, endian: str = '>'):
    """
    The structure of skin file: header, mid, tail
    header: ncells, (i1seed, i2seed, i3seed)(new version), int32
    mid: [z, y, x, likelihood, strike, dip, ...] * ncells, float32
    tail: [above, below, left, right] * ncells, int32

    load a skin file into two parts: mid and tail

    Parameters
    -----------
    filename : str
        skin file name
    endian : str
        little endian is '<', big endian is '>'

    Returns
    --------
    mid : array-like
        shape is [ncells, 12] or [ncells, 9] (old version)
    tail : array-like
        shape is [ncells, 12] or [ncells, 4] (old version)
    """
    d = np.fromfile(filename, dtype=np.int32)
    if endian == '>':
        d = d.byteswap()
    if (d[0] * 24 + 4) == len(d) or (d[0] * 13 + 1) == len(d):
        ncell = d[0]

        nele = (len(d) - 4) // ncell
        if nele == 24:
            nattr = 12
            nnigb = 12
            mid = d[4:4 + ncell * nattr].view(np.float32).reshape(ncell, nattr)
            tail = d[4 + ncell * nattr:].reshape(ncell, nnigb)
        else:
            nattr = 9
            nnigb = 4
            mid = d[1:1 + ncell * nattr].view(np.float32).reshape(ncell, nattr)
            tail = d[1 + ncell * nattr:].reshape(ncell, nnigb)

    elif (2 + d[0] * 6 + d[1] * 10) == len(d):
        """.cig format"""
        nctrl = d[0]
        ncell = d[1]
        start = 2 + nctrl * 6
        mid = d[start:start + ncell * 6].view(np.float32).reshape(ncell, 6)
        tail = d[start + ncell * 6:].reshape(ncell, 4)
    else:
        raise RuntimeError("Unknow file type")

    return mid, tail

Let me know if this implementation make sense, will create PR to integrate this changes