Added a test for concat_niimgs
Optimized memory usage of concat_niimgs

Dropped usage of np.concatenate(), reducing memory usage by a factor
of 3. A test for inconsistent shapes has been added, with a clear user
error message.

Execution time of concat_niimgs() is similar, possibly slightly
shorter than for the previous version.
Merge pull request #46 from pgervais/concat_optimization

Reduced memory consumption for concat_niimgs