Adding dec files for study of Sigma->p+ nu nubar and backgrounds.
Added 5 dec files for a study of sigma+ -> p+ nu nubar. This includes the signal along with four background processes of interest; sigma+ -> p+ pi0, sigma+ -> p+ gamma, sigma+ -> p+ mu+ mu- and sigma+ -> n pi+. All are constrained by a Lambda_c+ -> Sigma+ pi+ pi-. Made following the contribution guide for the DecFiles package.