Automatically detect "filler" values

[wfvining]

Many data sets have filler values in them representing either times when the sensor or data logger was not working (i.e. 999, -999). When we know what the filler values are it is fairly straightforward to detect/remove them; however, we may not always know what values are used as "filler." We should add some methods for automatically identifying filler-values in quality.gaps.

One method for automatically identifying filler-values could be to look for identical outliers that occur repeatedly throughout the data. There are almost certainly other ways to do this. This issue can serve as a solicitation for these methods if anyone has any suggestions.

[rjstephens]

A possible method could look for the common filler values (-999, 999, "null", etc) and return an error if none or more than one is found before going onto your suggestion of finding the most common value.

[wfvining]

@rjstephens Definitely a good idea to implement the simple filter first. I haven't gotten around to it yet. I like your suggestion to look for multiple occurrences of each value.