add: masking details

Add entities to model:
* custom mask as an input of `model fitting`
* mask generated by SPM as an output of `model fitting`
* another custom mask as an input of `contrast estimation`
delete: `custom mask 2`
Add final mask and model small volume correction

As suggested by @nicholst:
- Change the name "Search Space", which is 'used by' the "Inference"
activity, to be "Sub-Volume".
- Add "Search Space" as an entity 'generated by' the "Inference"
activity.
remove arrow between mask and contrast estimation.

As pointed out by @satra on the DDWG-NIDASH call on 10th Feb.: SPM
generates the mask but use NaNs for masking.
"mask" is now used by "Inference" and "Contrast estimation"

As agreed on NIDASH-DDWG call, March 3rd, 2014.
Merge pull request #27 from cmaumet/spmUp_masking

Modelling of masking