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.