Found problem in code. setExercise method of Shimmer class

calls update, but update will throw an error if there is an invalid filename.

Thus, if you setExercise before setFilename in a Shimmer instance, you'll
have a bad time.
stashing commit. code breaks on testShimmer again. I'm trying

to examine the Shimmer class and clean up the code so that it should only
throw exceptions in the beginReadingData method of Shimmer
Shimmer class working again. Throws a derivative class of

runtime_error if unable to read data from file or if an exercise is NULL.
Also able to specify either the exercise or the filename for the Shimmer
in either order