Adding SpatialExperiment tutorial

[cansavvy]

Summary

⚠️ This is stacked on to #22 so that should be reviewed first

Here I'm adding a tutorial (still a draft) for how to use SpatialExperiments with spatialTIME package.

For review

1. Clone this repo and add fhdsl/spatialTIME as a remote.
2. Checkout this branch
3. Run devtools::load_all()
4. Then go step by step through vignettes/spatialexperiment.Rmd

It certainly needs more clarity in some places -- some of these places will be addressed by slides but we may need reminders or more simplistic explanations.