spelling out methods names

[jkanche]

but still supporting the short notations for backwards compatibility

In SpatialExperiment class:

• get_image alias to get_img
• add_image alias to add_img
• remove_image alias to remove_img
• image_source alias to img_source
• image_raster alias to img_raster
• rotate_image
• mirror_image

in the SpatialImage classes:

• image_source alias to img_source
• image_raster alias to img_raster
• rotate_image
• mirror_image