`__contains__`, `overlaps`, `overlap_rate`, `intersection`, `get_area` methods provide errors/wrong results for geos AreaDef / SwathDef

Problem description

The methods __contains__, overlaps, overlap_rate, intersection, contains, get_area implemented in BaseDefinition class employs a polygon defined by the centroid of the pixel corners of the SwathDef / AreaDef to perform the geometrical computations.
While this works correctly for many classical AreaDefinition, for:

• Geostationary AreaDef all methods crash (because of Inf values at the corners) (see also #393, #268) 3https://github.com/pytroll/pyresample/issues/393
• SwathDefinition, the corners of lats/lons arrays does not provide meaningful values in the case of ascending and descending orbits within SwathDefinition ...

Code example for GEO AreaDef

import satpy
geometry_def = satpy.resample.get_area_def("goes_east_abi_f_1km")
geometry_def.corners   # --> Illegal (lon, lat) coordinates: (inf, inf)
geometry_def.outer_boundary_corners # --> Illegal (lon, lat) coordinates: (inf, inf)
geometry_def.get_area() # --> Illegal (lon, lat) coordinates: (inf, inf)
geometry_def.overlaps(geometry_def) # --> Illegal (lon, lat) coordinates: (inf, inf)
geometry_def.overlap_rate(geometry_def) # --> Illegal (lon, lat) coordinates: (inf, inf)
geometry_def.intersection(geometry_def) # --> Illegal (lon, lat) coordinates: (inf, inf)
(10,20) in geometry_def # __contains__ method  # --> Illegal (lon, lat) coordinates: (inf, inf)

Proposed solution:

• Definition of boundary method for SwathDefinition and AreaDefinition to retrieve AreaBoundary class
• Definition of SphPolygon from AreaBoundary using self.polygon()
• Enhancement of spherical.py geometrical operations
• Upgrade of BaseDefinition methods to use self.boundary().polygon().<method>(...)

Draft PR coming in the next days ...

Start with the BaseDefinition methods listed in the issue and inspect spherical.py, then trace the proposed AreaBoundary and polygon() entry points for AreaDefinition and SwathDefinition. Done means the listed geometry methods no longer fail on geostationary AreaDef inputs or produce wrong results for the described SwathDefinition cases.