Tidy up some of the lte funcs

* Use optional function in step_to_midpoint funcs for dy est

* Use imr_f_approximation_error in imr_lte
Add function dts2ts
Add generate_predictor_dt_func()

* Create functions to calculate predictor dts from corrector ts
  (using sympy expr as input)
Modify generate_predictor_scheme() to use last few commit's functions
Add contact details to readme