Cleanup Fixed_Time_2PL, use cython methods (like in Fixed_Time_2PL_SAM), make sure both classes are consistent

[lzkelley]

See also: #43

[ ] Use cython methods in Fixed_Time_2PL
[ ] General cleanup of Fixed_Time_2PL
[ ] Implement tests to check for consistency between Fixed_Time_2PL and Fixed_Time_2PL_SAM