Create grand mean map entity

As discussed at the hackathon/HBM 2014 with @nicholst, @gllmflndn,@jbpoline, @chrisfilo and @tiborauer (see #68) and then with @khelm and @satra at #70.

This is a proposal to create a grand mean map, output of `Model Parameters Estimation`. This will be useful as a diagnostic of the data scaling.

 (The `medianIntensity` attribute from `Data` is also deleted in this pull request and replaced by the `maskedMedian` attribute of the `Grand mean map`).