The function `_reduce_over` has `args` and `kwargs` as inputs, but never
use them. This commit changes `_reduce_over` so that `args` and `kwargs`
are passed to `squeeze` in `_reduce_over`.

The commit removes `update_long_name_average!` and replace it with
`update_long_name_generic!` so that it can be used by any other
functions that do some kind of reduction on data.

This commit adds Var.integrate_lonlat, Var.integrate_lon, and
Var.integrate_lat for integrating `OutputVar` with respect to longitude,
latitude, or both. Additionally, this commit adds a Numerics module to
decouple the numerical implementation of integration and the
construction of an OutputVar. The numerical implementation is
rectangular integration using the midpoint rule when the points are
equispaced which assume each point correspond to a midpoint of a cell
and using the left endpoint rule when the points are not equispaced.