Delft 3D Turbulent Intensity Function

[browniea]

Describe the bug:

The Turbulent Intensity Function runs slow when a large data set in input.

To Reproduce:

d3d_data = netCDF4.Dataset('tanana50_map.nc') data
x_centerline = np.linspace(xmin, xmax, num=100)
y_centerline = np.mean([ymin,ymax])
z_centerline = np.mean([zmin,zmax])
centerline_points = d3d.create_points(x_centerline, y_centerline, z_centerline)

TI=d3d.turbulent_intensity(d3d_data, points= centerline_points ,intermediate_values = True)

Expected behavior:

Adding a slicing function to limit the data size processed through the function will speed up function.

[ssolson]

Close #375