Multi-camera systems for rehabilitation therapies: a study of the precision of MicrosoftKinect sensors<FootNote> Project partially supported by Spanish Ministerio de Economíay Competitividad/FEDER (Nos. TIN2012-34003 and TIN2013-47074-C2-1-R) and FPU Scholarship (FPU13/03141) from the Spanish Government </FootNote><FootNote> A preliminary version was presented at the 13th International Conference on Practical Applications of Agents and Multi-Agent Systems, June 3–4, 2015, Spain </FootNote>