Check for null/empty values before clustering

Having null or empty value is a case not considered in the
algorithm. This will lead to a infinite loop since some
features will not add to any region.
If any of the attributes for the clutering or threshold appear
as null/empty values, the plugin will display an error with the
first feature that failed this condition.
Check for null/emtpy values in dialog box

Before we let the user start the execution we will check for null
values every time that the attributes change. This is true for
both the clustering attribute and the threshold attribute.

- At the moment the dialog will display a dialog box, but this
could change to a message in the message bar.
plugin version, managerial commit

The changes in this version resolves #34, alerting the user about
null/emtpy attributes.
Merge pull request #37 from clusterpy/null_values

Null values