Issue #71 - Allow Relating units together with Multipliers

[robons]

This PR serves to close Issue #71.

However, instead of allowing the user to specify a multiplier against each observations, we are allowing the user to define a new unit which is related to an existing one by a multiplying factor. This has the same effect of allowing us to define the inter-relationships between units which vary only by their magnitude.

See here for an example of how the new units can be defined relative to existing units.

See here for an example of the RDF which is output.

Uses some qudt along with the om2 ontology since the qudt ontology itself isn't sufficient to specify the multiplying factor relationship between two units in a low-maintenance way.

[rossbowen]

I don't see that we need to choose between OM2 and QUDT, it seems reasonable we could make use of both.

For units like MillionGBP I'd still like to specify a qudt:converstionFactor. It feels inconvenient to have to go via the base unit. (Though MillionGBP is one we'd end up defining centrally anyway).

How would a user specify something like "megalitres per day"?

[robons]

I don't see that we need to choose between OM2 and QUDT, it seems reasonable we could make use of both.

I'm not entirely sure what you're suggesting here... I think we are using both.

For units like MillionGBP I'd still like to specify a qudt:converstionFactor. It feels inconvenient to have to go via the base unit. (Though MillionGBP is one we'd end up defining centrally anyway).

I think if we take this approach we might want to take a more organised and systematic approach to the QUDT ontology and ensure we define all the related quantityKinds, etc. that we think we should be doing. It's probably something we would want to do more thoroughly than the quick-and-dirty approach that we're taking here to make it easy for users to do something as expressive as possible quite quickly.

How would a user specify something like "megalitres per day"?

i think they'd base it off the m^3 per day qudt unit (http://qudt.org/vocab/unit/M3-PER-DAY) and provide the conversion factor of 10^3.

1 megalitre = 10^6 litres = 10^6 * 10^-3 m^3 = 10^3 m^3

[CharlesRendle]

@santhosh-thangavel and I have worked through the changes made by this PR and have come up with some questions regarding the gaps in our combined knowledge.

[santhosh-thangavel]

@ Rob, Thanks for the answers.

[rossbowen]

Cheers @robons, would be good to get qudt:conversionMultiplier as an optional attribute on NewQbUnit.

[robons]

Cheers @robons, would be good to get qudt:conversionMultiplier as an optional attribute on NewQbUnit.

@rossbowen I've added the qudt:conversionMultiplier functionality there. I've made it codependent on providing the quantity kind that the unit is part of since the qudt:conversionMultiplier only allows automated conversion between units when part of a quantityKind family.

Happy with the changes?