#38 vrt.copy() copies adsVRT as well.
#38 vrt.copy() copies all attributes whose name starts with 'sub_'. Modify mappers which has vrt attributes.
#38 all subVRTs are in vrt.subVRTs dictionary.
Merge branch 'issue43-nansat-crop' into issue38
#38 Nansat.add_band: add new super VRT and add band to this supVRT
#38 fix docstrings, fix some bugs related to VRT.get_projection, restore VRT.get_resized_vrt, move vrt and subVRTs from __init__ to class VRT, adding examples
#51 #43 use interactive PointBrowser for electing extent of the crop
#51 #43 add lonlim, latlim to Nansat.crop