add scale/resolution to spatial section

[pvgenuchten]

resolves #189

• introduces the spatial section on iso19139 import, including datatype property

To discuss:

• OWSLib loads distance and uom both as array, here i convert it to [{'distance': 5,'uom': 'm'},...]
• is mcf['spatial'] the most relevant section? Alternatives are mcf['identification'] or mcf['content-info']

[pvgenuchten]

is this an ok approach, or we expect this property to be always returned by owslib?

[pvgenuchten]

discussed at osgeo codesprint vienna, distance and scale are arrays, similar to iso19139, makes sense (min,max)