Add an option to build the bounds from the spatial extent of a source file

[alexamici]

Nice to have:

• support both raster and vector data via GDAL/OGR
• WGS84 bounds must be always larger or equal than the projected extent of the source
• let the user define the margin in a flexible way
  • percent
  • decimal degree
  • metric (not easy)
  • DEM pixels

[alexamici]

Current implementation is based on rasterio and fiona that are added to the dependencies. In principle it is possible to use only GDAL python bindings, with a small cost in easiness of use and a (smaller IMO) advantage of lighter dependencies.