Fixed problem with cummulative_rms which did not correctly calculate the RMS for frequency vectors which were changing when the frequency vector was flipped

Fixed problem where gps_start and gps_stop are sometimes not ints when it hits nds2.  Committing version 0.1.6 for nds2utils.

Commiting version 0.1.5, which fixed find_channels to print the time at which find_channels is looking at

Fixed issue with nds2utils/make_interactive_svg.py, which was using a depreciated function for xml tree.getchildren() which was not supported in future versions of python.  Replaced with list(tree) which is backwards-compatible

Added functionality for taking only a single PSD, CSD, or TF + coherence as needed.  Can save time with large dictionaries

Added check_loaded_data_dict() function to data_utils.py, which checks the data_dict loaded versus a list of channels, and also does some basic checks of the data and if the data_dict is None.  Also put some tests of this function in test_data_utils.py

Added some new functions to check the duration and bandwidths from the user, and adjust them if necessary.

Making some small changes to dtt_averages() for when the user makes a bad request for number of averages (number of averages should always be int)

Added first pytest file test/test_data_utils.py to eventually test the functions in nds2utils/data_utils.py