Thisworkwas supported by CNPq (ConselhoNacional do Desenvolvimento
da Pesquisa), CAPES (Coordenac¸ ão de Aperfeic¸ oamento
de Pessoal de Nível Superior) and FAPEMG (Fundac¸ão de Amparo a
Pesquisa de Minas Gerais). Danielle Marra de Freitas Silva received
scholarship from CAPES. The authors wish to thank Ms Aparecida
Guerra de Jesus for technical assistance. She received scholarship
from FAPEMG.