Use Precise Map sub-type as type for original maps

Following #126, for each file created as part of a NIDM-Results export we have two entities representing:
- the newly created file (blue entity in the example);
- the original version of this file which was created by the analysis software package (green entity in the example).

Currently the original file entity always has the generic type `nidm:Map`. 

Example (StatisticMap):
![Type of original entities: current](https://cloud.githubusercontent.com/assets/5374264/4440646/fcbdb306-47c7-11e4-9471-1b72313333e7.png)

This is a proposal to use a more specific typing for entities describing original files (cf. update in yellow):
![Type of original entities: proposed update](https://cloud.githubusercontent.com/assets/5374264/4440645/f7c9b6f6-47c7-11e4-8b7b-204991c858cd.png)

The original file and the newly created file would then have the same type (except in FSL where the `ContrastStandardErrorMap` was derived from a `VarCope` map).