Alternative reproject method, better consistency for window selection + additional features at load

[deastwoo]

changes summarized:

• fixed: resampling and reprojection to different CRS now with rasterio reproject function
• fixed: more precise window selection and dimension definition to avoid rounding errors which occurred occasionally
• fixed: io window read can handle different projections and transforms and this is consistent with save_as_geotiff
• 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

[coveralls]

Coverage decreased (-0.6%) to 78.835% when pulling 019b2c4070cbb17ff8999fae779fdc91cb8f8873 on alt_reproj into 7329bd65faf4cad55cd6281cea16aaf21b572b08 on master.