Re-organize the structure of geoutils

[adehecq]

We need to re-think the organization of some submodules of geoutils and write a clear plan for what functionalities we aim to include and where, write a release schedule. A non exhaustive list of questions and things to be addressed:

• spatial_tools: probably rename and clarify the functionalities to be included in here (should filters be added here?). Move to the raster submodule?
• same for projtools
• move satimg: see Move SatImg class to georaster/ folder #260

Some related issues:

• Merge merge_bounds function from projtools and spatial_tools #255
• Update the release schedule #200
• Check what tools can be imported from pygeotools #51
• Consider adding Savitsky-Golay algorithm to Raster class #26.