add vegetation transition plots as default outputs?

[achubaty]

Following up on previous discussions, and new functionality in LandR (PredictiveEcology/LandR#107), it'd be nice to add these as default outputs of this module. They help visualize the transitions among vegetation 'types' -- which can be by species or by species-group (e.g., conifer/deciduous/mixed). Very useful as debugging/diagnostic plots.

Because these read in all of the pixel values from multiple vegTypeMaps, storing and summarizing the data for these, and producing the plots requires quite a bit of additional memory and computation time. The LandR functions to create these are designed to read values from disk, which reduces the memory footprint substantially.

To add these to the module:

1. What makes the most sense as a default? All species-level transitions, conifer/decid/mixed (if mixedType > 0)? These are currently the easiest to get at, because they use the vegTypeMap produced by the module.
2. What time intervals should be used for producing these? Should we add a new module parameter to allow the user to adjust this?
3. Storing all the pixel-level data for each time slice (like we do for the summaryBGM plots) in memory adds a lot of overhead, so we need to ensure the vegTypeMap objects all get written to disk.