Merging updates from the development branch to create agcounts 0.3.0

Merging updates from the development branch to create agcounts 0.3.0

Major updates since agcounts 0.1.0 that are included in this version:

Create a new function called calculate_counts that can read in raw data from a data frame. This allows the user to read in data from R that doesn't need to be a GT3X format. It does need to have time, X, Y, and Z variables as well as start_time and stop_time attributes.
Automated the detection of sampling frequency so it doesn't need to be entered by the user.
Forced UTC time zones throughout the package instead of using America/Chicago. This closes issue #3 and #4.
Added dateTimeAs = "write.csv" to data.table::fwrite function to output correct date format when write.file = TRUE. This closes issue #8.
Added Paul R. Hibbing as a package contributor.
Added unit testing to compare data from the agcounts package with Actilife and to assess different stages of the pipeline. @paulhibbing you may have some suggestions here as this was my first time adding automatic testing. This closes issue #2.

bhelsel / agcounts