Configurable data folder

[benjamin-heasly]

We currently have configurable folders for toolboxes, projects, and local hooks. This is sufficient for many toolboxes and projects.

For others, it might be useful to have a separately configured "dataRoot" folder, where toolboxes and projects can look for input data and/or write output data. This would allow toolboxes and projects to establish conventions for how their data should be organized, but without having to worry about where the data were located on the local file system.

Implementing this would have a few parts:

• new comments in sampleStartup.m to help users specify their own data folder as a Matlab preference
• new code and documentation in tbParsePrefs for a new dataRoot parameter
• a new utility tbLocateData() to obtain the currently configured data root as a one-liner. This should accept a prefs struct, similar to tbLocateToolbox() and tbLocateProject().
• tbLocateData() should be mentioned in the wiki page about writing local hook templates, as a way to dynamically choose sensible defaults