wifi-heatmap-thresholds: add required HeatMapGenerator.__init__ parameters

[kledom]

The wifi-heatmap-thresholds script is currently not working due to missing parameters. Since the parameters are irrelevant for the functionality of the script I added the default values.

Contributor License Agreement

By submitting this work for inclusion in wifi-survey-heatmap, I agree to the following terms:

• The contribution included in this request (and any subsequent revisions or versions of it) is being made under the same license as the wifi-survey-heatmap project (the Affero GPL v3, or any subsequent version of that license if adopted by wifi-survey-heatmap).
• My contribution may perpetually be included in and distributed with wifi-survey-heatmap; submitting this pull request grants a perpetual, global, unlimited license for it to be used and distributed under the terms of wifi-survey-heatmap's license.
• I have the legal power and rights to agree to these terms.

[jantman]

I believe this has been resolved in subsequent releases.