##CodeBook
- act_labels: table with columns "activity_id" and "activity" which maps numbers to descriptions.
- features: contents of features.txt
- data: local variable used in function read_data() for holding the data of a given data set (test or train)
- all_data: data of both train and test data
- data_per_act: this is "all_data" grouped by "activity", holding the mean() values of all columns.
- project-result.txt: the file where the result table is exported