Remove basemap shiftgrid

[JessicaMeixner-NOAA]

ww3tools/gridSatGlobal_Altimeter.py and ww3tools/prepGridMask.py are using the shiftgrid from basemap
from mpl_toolkits.basemap import shiftgrid
which can be replaced with another shiftgrid function (quote from @ricampos in #61)