Add month symbology

[valpasq]

Add a symbology field that would enable symbolizing observations by month.

Mockup using MATLAB:

Colors could definitely be improved, but should be somewhat cyclic like the hsv colormap so that DOY 1 is more similar to DOY 365 than DOY 185.

[ceholden]

We discussed the implementation of this at our lab group "hacky hour" yesterday.

Adding the month of the observation to the available metadata is very easy and only requires 4-5 lines of code in one of the time series drivers (e.g., probably best done in StackedTimeSeries so subclasses inherit this widely useful metadata item).

The only hangup is that TSTools' metadata symbology is tailored for qualitative, categorical data and months, denoting the progression through a year, are best visualized using a sequential colormap. We're fine on the categorical data front, but we would have to somehow specify that this metadata item should use a sequential colormap.

Proposed solution to allow sequential colormaps is to add an additional parameter that describes each metadata item (sequential/categorical). See ticket #78