MINC specific convenience functions

A few routines are provided to deal with some of the minc structures. miattput_pointer and miattget_pointer put/get a pointer to a variable attribute. miadd_child helps maintain the hierarchy of variables by handling the MIparent and MIchildren attributes of two variables. Finally micreate_std_variable and micreate_group_variable create some of the standard variables and fill in a few of the default attributes.