Changed the way compatibility works

Loading compatibility functions is now done by checking if the function
already exists rather than testing the version of Matlab. Also, padarray
was moved to compatibility.
Refactor accumulateOde with bug fix

The god function accumulateOde had a bug in it when using delta while
integrating forward. This bug did not affect any of the current top
level functions. To fix this and prevent problems in the future, the
function was refactored into four different functions, representing the
four combinations of forward/reverse and continuous/discrete.