RuntimeWarning in some examples due to `rthEllipse` components

[jwbishop]

/Users/ldtoney/repos/lts_array/lts_array/classes/lts_classes.py:507: RuntimeWarning: invalid value encountered in cdouble_scalars
D = np.sqrt(foo + (a * b - 2 * c - a2 * a / 4) / R)
/Users/ldtoney/repos/lts_array/lts_array/classes/lts_classes.py:508: RuntimeWarning: invalid value encountered in cdouble_scalars
E = np.sqrt(foo - (a * b - 2 * c - a2 * a / 4) / R) # ...