Documentation needed for new features from PR #64

[brittonsmith]

PR #64 introduced a number of new features that need documentation. These include:

• added: up/down-scaling by scalar factor at load
• added: nodata value can be defined at load to use internally (sometimes useful when geotiff's metadata doesn't have a consistent nodata value)
• added: select your rasterio resampling method at load (not always nearest neighbour anymore)
• fixed: coordinate reference system to work in can be defined at load
• added: polygons allows you to map a list of polygons/shapefiles to create a list of yt polygon selections
• enhanced: field_map now works for multiple files
• added: get_field_as_raster_array function outputs window in 2d grid as callable