Funding for this research was provided by:
Fundação Nacional para a Ciêencia E Tecnologia (PEst-OE/MAT/UI0006/2014)
Received: 2 December 2016
Accepted: 15 June 2017
First Online: 30 June 2017
Compliance with ethical standards
: The authors certify that there is no actual or potential conflict of interest in relation to this article.