Add new functions:
1. _cm_contrast_2cmap and cm_contrast_red2green for makding
  unsymetric contrasting cmap
2. change Set_Cbar_Label_Parallel to colorbar_set_label_parallel
3. Axes_get_bounds for a group of axes
4. _get_axesrec_by_pos_offset and Axes_add_axes to add new axes
  in relative to a group of axes
Add in Pdata.Mdata and bmap.py
the show_colorbar keyword to allow suppressing of colorbar in
mapcontourf
add more biomass levels in different units
minor change in documentation of gnc.Ncdata.get_pftsum