finished smoothtemps, commented timediffs, add flexibility to ttmodel

smoothtemps.py

smoothedTTs():
- return value
- takes step size and time range as input arguments

timefiffs.py

- enhanced comments

ttmodel.py

- enabled k and p1 to be specified as command-line parameters
- enhanced plot annotations
- enhanced Usage comments and modified other comments
overhauled ttmodel.py; use scipy.optimize.minimize