Add a compositor that can use static images

[pnuu]

Feature Request

In #460 it has been discussed that a compositor that can use a static image as a background would be very useful.

I've been thinking about this "BackgroundCompositor" for a while even before the issue was opened, and already have some ideas. In fact this might end up being two compositors:

1. StaticImageCompositor - read an image and use it as is (L(A) or RGB(A)), or as a channel in a composite (L(A))
2. BackgroundCompositor - use a static image (either directly or using StaticImageCompositor) and overlay another composite on top of it (blend using A channel of the top-image as weight)

With these and already existing DayNightCompositor and CloudCompositor it would be possible to create images having:

• Earth at Night as a background and overlay of clouds
• mix of Earth at Night and Blue Marble as background and overlay of clouds
• a static map as a background and overlay of clouds

[pnuu]

Some things to consider when implementing:

• how to handle start_time for a static data
• geotiff can be read with generic_image reader and has CRS info
• other image formats with the CRS info given in config YAML?
• use as is if the shape of the target geo def matches with the static image shape

[djhoese]

For the static image loading from YAML, would it be useful to use PIL's resize to stretch the loaded image to the input dataset (https://stackoverflow.com/a/451580/433202)? Or would that cause issues if someone wanted to resample their data first, then use the static image on the resampled data where they would probably match size.

Which makes me think maybe a keyword argument for the compositor to say that the static image has to be the same shape as the input data? That way you could turn this on/off.