'datetime' compatibility

[dr-parker]

For older versions of MATLAB, comment/modify the call to datetime('now') in the following files:
\ARobotarium.m
date = datetime('now');
this.file_path = strcat(this.file_path, '_', num2str(date.Month), '_', num2str(date.Day), '_', ... num2str(date.Year), '_', num2str(date.Hour), '_', ... num2str(date.Minute), '_', num2str(round(date.Second)), '.mat');

May also be necessary to repeat in the following file:
\robotarium-matlab-simulator-master\utilities\misc\unique_filename.m

date = datetime('now');
filePath = strcat(file_name, '_', num2str(date.Month), '_', num2str(date.Day), '_', num2str(date.Year), '_', num2str(date.Hour), '_', num2str(date.Minute), '_', num2str(date.Second), '.mat');

Updating these files to make backwards compatible does not appear to be critical, but requires some use of the date, now, clock, and/or datenum functions from older MATLAB versions.