Refactor null.perms() with additional style changes and improved function generality.  Small change to vario_bisect() to make it more consistent with vario().
Refactor null.perms() with additional style changes and improved function generality.  Small change to vario_bisect() to make it more consistent with vario().

weecology/mete-spatial,weecology/mete-spatial,weecology/mete-spatial,weecology/mete-spatial