Remove fsl:varCope entity and keep only nidm:ContrastStandardErrorMap

In the current version of NIDM-Results for FSL, `ContrastStandardErrorMap` is derived from a `ContrastVarianceMap` that is generated by `ContrastEstimation`:
![](http://nidm.nidash.org/specs/img/nidm-results-model-fitting-fsl.png)

To simplify querying across software (SPM, FSL), this pull request is a proposal to remove the `ContrastVarianceMap` entity as in:
![](https://cloud.githubusercontent.com/assets/5374264/3508964/969c3e64-0693-11e4-874a-05143e8b238a.png)

If this pull request is agreed upon, we will need a mechanism to link the ContrastStandardErrorMap to the original varCope file. This will be updated following discussions at #108.