HEMS example: set --ensure-positive explicitly as it cannot be used as a flag right now

[nhoening]

PR #180 should move the HEMS example to using forecasts via the API, but in the meantime v0.31 might have introduced a regression in the CLI.

--ensure-positive is intended to be a flag, but we define it via Marshmallow now, and click fails before Marshmallow validates. We have to use --ensure-positive True.

So consider this a quick fix. The CLI should be fixed, but before that is live, we should finish #180 anyway.

This PR might have to stick to the old behavior for FM < v0.31, I am not sure.

[coveralls]

Pull Request Test Coverage Report for Build 22845759300

Details

• 0 of 0 changed or added relevant lines in 0 files are covered.
• No unchanged relevant lines lost coverage.
• Overall coverage remained the same at 97.815%

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Totals
Change from base Build 22532251905:	0.0%
Covered Lines:	582
Relevant Lines:	595

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