Pull #168 from SpiritOffice : Fix for Circularize function

if you want to change your hyperbolic orbit to circular orbit at some
altitude :

In old version it says you can't circularize, because of this statement:
"circularizeAltitude < o.ApA". o.ApA is negative.
I have just added the same test as in NextTimeOfRadius() function. It
should never throw ArgumentException.