Add CoP calculation

- Add sum, divide, subtract operations
- Change datasets API
- Simplify dataset calculation
Merge pull request #61 from MastodonC/feature/cop

Feature/cop
Fix lower and upper bounds for the sensor.
Merge pull request #62 from MastodonC/feature/fix-bounds

Fix lower and upper bounds for the sensor.
Fixed display of property id in table.
Merge pull request #63 from MastodonC/feature/property-details

Fixed display of property id in table.
Use dirty dates for datasets.
Initialise dirty dates for datasets and other fixes
Fix typo in render-item(s).
Add multiply operation.
Merge pull request #66 from MastodonC/feature/calculated-fields

Feature/calculated fields
templates wip - some tests.
more decisions implemented.
Add LZ4 library to avoid annoying warning.
More tests.