ENH: Add Active Registration Model metrics

[kaspermarstal]

This PR adds an intensity penalty metric and a shape penalty metric inspired by Active Appearance Models. The metrics are disabled by default.

Kasper Marstal and Stefan Klein "Active registration models", Proc. SPIE 10133, Medical Imaging 2017: Image Processing, 101330Y (24 February 2017); https://doi.org/10.1117/12.2254356).

[kaspermarstal]

Mhm, I disabled the metrics by default because of the external dependency on boost. Seems like it still tries to find boost even though the metrics are disabled. Will fix this.