added new property 'mask_baddata' to GeoArray; some bugfixes and further developments; new dtypes package

- added package 'dtypes' with submodule 'conversion' for performing data tyoe conversions

geo.raster.reproject:
- warp_ndarray(): input data types that are incompatible to GDAL are now automatically transformed to a GDAL compatible data type

io.raster.gdal:
- get_GDAL_ds_inmem(): added automatic data type conversion if not compatible to GDAL

io.raster.GeoArray:
- GeoArray:
    - __init__():
        - bugfix for rejecting subclasses of GeoArray
        - added attribute '_mask_baddata'
    - added property 'mask_baddata' + setter
- added class 'BadDataMask' (subclass of GeoArray)

- updated __version__