Fix operator id numbering

[oxixes]

Proposed changes

The operator ID is represented as a string when it should not, which causes the IDs of new operators to just be old one plus a one, leading to IDs like "9711111111111". This PR fixes that issue.

Types of changes

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• Bugfix (non-breaking change which fixes an issue)
• New feature (non-breaking change which adds functionality)
• Breaking change (fix or feature that would cause existing functionality to not work as expected)

Checklist

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• I have read the CONTRIBUTING doc
• I have signed the CLA
• I have added tests that prove my fix is effective or that my feature works
• I have added necessary documentation (if appropriate)
• Any dependent changes have been merged and published in downstream modules

Further comments

None

[coveralls]

coverage: 88.222% (+0.01%) from 88.21%
when pulling 967b89b on oxixes:fix-operator-numbering
into 2a25766 on Wirecloud:develop.