Topotools

Fixed some typos and fixed topotools.py so that negative topo_type is supported (in which case Z>0 means depth below sealevel rather than elevation above, so Z must be negated).

Other comments:

Topography.plot:  
Fixed so clim is symmetric about 0.
I found it useful to be able to specify cmap_neg and cmap_pos separately for
cases where the range of topo values and bathy values are very different,
e.g. you want to plot bathy colors for -4000 < Z < 0 but topo colors over a
smaller range 0 < Z < 100, say.  Now if you call topo.plot(limits = (-4000,100))
then part of the bathymetry will be green, not brown.

Several other plot options are no longer supported.

The old topotools.topo2writer  is used in maketopo.py in many examples.  We need to add this functionality.