Deprecation warnings with latest numpy (1.10)

[fmaussion]

From the travis log:

......../home/travis/build/OGGM/oggm/oggm/prepro/gis.py:209: DeprecationWarning: non integer (and non boolean) array-likes will not be accepted as indices in the future
  glacier_mask[y, x] = 0  # on the nunataks, no
/home/travis/build/OGGM/oggm/oggm/prepro/gis.py:211: DeprecationWarning: non integer (and non boolean) array-likes will not be accepted as indices in the future
  glacier_mask[ext_yx] = 1
/home/travis/build/OGGM/oggm/oggm/prepro/gis.py:212: DeprecationWarning: non integer (and non boolean) array-likes will not be accepted as indices in the future
  glacier_ext[ext_yx] = 1
/home/travis/build/OGGM/oggm/oggm/prepro/centerlines.py:457: DeprecationWarning: non integer (and non boolean) array-likes will not be accepted as indices in the future
  zoutline = topo[ext_yx]
/home/travis/virtualenv/python3.4.2/lib/python3.4/site-packages/skimage/graph/mcp.py:89: DeprecationWarning: using a non-integer number instead of an integer will result in an error in the future
  return m.traceback(end), costs[end]
/home/travis/build/OGGM/oggm/oggm/tests/test_prepro.py:223: DeprecationWarning: non integer (and non boolean) array-likes will not be accepted as indices in the future
  my_mask[ext_yx] = 1
/home/travis/build/OGGM/oggm/oggm/tests/test_prepro.py:252: DeprecationWarning: non integer (and non boolean) array-likes will not be accepted as indices in the future
  kien_mask[ext_yx] = 1
../home/travis/build/OGGM/oggm/oggm/prepro/centerlines.py:577: DeprecationWarning: using a non-integer number instead of an integer will result in an error in the future
  heigts.append(topo[head.y, head.x])
.../home/travis/build/OGGM/oggm/oggm/prepro/geometry.py:471: DeprecationWarning: non integer (and non boolean) array-likes will not be accepted as indices in the future
  costgrid[y, x] *= cost_factor
/home/travis/build/OGGM/oggm/oggm/prepro/geometry.py:683: DeprecationWarning: using a non-integer number instead of an integer will result in an error in the future
  widths[i0:i1] = np.NaN
./home/travis/build/OGGM/oggm/oggm/prepro/climate.py:137: DeprecationWarning: converting an array with ndim > 0 to an index will result in an error in the future
  time = time[p0:p1+1]
/home/travis/virtualenv/python3.4.2/lib/python3.4/site-packages/netCDF4/utils.py:359: DeprecationWarning: converting an array with ndim > 0 to an index will result in an error in the future
  beg, end, inc = e.indices(length)
.............

[fmaussion]

Addressed by #15