[Processing] GRASS7 v.distance (implements Redmine #12817)

[ghost]

This PR implements GRASS7 v.distance into Processing (implements #12817).

There are two outputs for this algorithm:

• A copy of the 'from' table with the updated attributes (for cat/dist/to_x/to_y/etc.).
• The vector with shortest lines between 'from' and 'to' vectors.

The attributes which will be updated by this algorithm should be created in the original 'from' table (they are not created on the fly). As you can update multiple fields at the same time, I haven't use ParameterTableField, which only permit the selection of one field. You have to write the list of fields to update by hand but there is, at least, a control of equality between columns number and 'upload' operations.

There is no direct output (stdout with -p and -a options).

This feature will be for QGIS 2.16.

Thanks to @volaya, @alexbruy , @gioman for reviewing...

[gioman]

thanks for adding this... for incredible as it can seem, we don't had a tool to do distance analysis (not even a plugin as far as I remember)...

[havatv]

@gioman, the NNJoin plugin was created because v.distance did not work in Processing, so there has been a plugin available.

[gioman]

@havatv thanks for the heads up!