Relax nrow and ncol reguirement for Full matrix.

When the user provides vectors as arguments to mxMatrix() and type is 'Full', only one of nrow and ncol is needed. This fix applies only to 'Full' matrices. The following matrices are all square: "Diag", "Iden", "Lower", "Stand", "Sdiag", "Symm". And "Unit" and "Zero" need both row and column information.