WIP: Contrast and contrastWeights

This proposal follows discussions with @nicholst, @nicholsn, Jessica and Karl on the [terms](https://docs.google.com/spreadsheets/d/16pC2cDsdxlzv2CzlNMtStqt5-xHwDEsU6MjZVxWhrE4/edit?usp=sharing).

Looking at `Contrast` (parent of `TContrast` and `FContrast`), we found out that it was difficult to define this term and to differentiate it from the definition of its attribute `contrastWeights`.

I would like to suggest that we adapt the changes made in pull request #36 to have:
<pre>
entity(niiri:contrast_id1,
[<b>prov:type = 'nidm:FContrastWeights',</b>
nidm:contrastName = "my contrast name" %% xsd:string,
prov:label = "My contrast description" %% xsd:string,
<b>nidm:tContrastWeights = "[1, 0]" %% xsd:string,</b>
<b>nidm:tContrastWeights = "[0, 1]" %% xsd:string,</b>
<b>prov:value = "[[1, 0], [0,1]]" %% xsd:string</b>)
</pre>

instead of:
<pre>
entity(niiri:contrast_id1,
[<b>prov:type = 'nidm:FContrast',</b>
nidm:contrastName = "my contrast name" %% xsd:string,
prov:label = "My contrast description" %% xsd:string,
<b>nidm:contrastWeights = "[1, 0]" %% xsd:string,</b>
<b>nidm:contrastWeights = "[0, 1]" %% xsd:string</b> ])
</pre>

This would still allow to relate an F-test to the corresponding T-Contrasts while aleviating the need to find a separate definition for the entity "Contrast" and its contrast vector attribute.

---
This pull request also includes terms updates as proposed in PR #39. (PR #39 should be reviewed first)