Add bandref to RasterInfoModel

[mmacata]

In GRASS GIS, r.info -gre now adds a new parameter bandref. As this was not included in the RasterInfoModel, requesting the RasterLayerResource leaded to an error:

exception": {
"message": "The model \"RasterInfoModel\" does not have an attribute \"bandref\"",
"traceback": [
"  File \"/usr/lib/python3.8/site-packages/actinia_core/rest/ephemeral_processing.py\", line 1732, in run\n    self._execute()\n",
"  File \"/usr/lib/python3.8/site-packages/actinia_core/rest/raster_layer.py\", line 314, in _execute\n    self.module_results = RasterInfoModel(**raster_info)\n",
"  File \"/usr/lib/python3.8/site-packages/flask_restful_swagger_2/__init__.py\", line 336, in __init__\n    raise ValueError(\n"
],
"type": "<class 'ValueError'>"
},

This PR adds the missing parameter to the model.

Tested with GRASS GIS 7.8 as well, so if r.info -gre does not return bandref, it is simply left out and response is successful.

[anikaweinmann]

Looks good. Thank you

[metzm]

Looks good, please merge!