[BUGFIX] Incorrect indexing to hub heights in Farm.coordinates()

[misi9170]

#846 introduced a bugfix to the Farm.coordinates() method, which was released and runs as expected in FLORIS v3.6. However, when develop was merged back into the v4 branch in preparation for the release of FLORIS v4.0, this line was missed in reducing from 5D to 4D data structures.

This PR fixes this issue by simply removing the now-redundant dimension in accessing hub_heights.

The following code demonstrates the error that is raised:

from floris import FlorisModel

fmodel = FlorisModel("inputs/gch.yaml")

# Changing the wind farm layout uses FLORIS' set method to a two-turbine layout
fmodel.set(layout_x=[0, 500.0], layout_y=[0.0, 0.0])

# Runs correctly
print(fmodel.get_turbine_layout())
print(fmodel.get_turbine_layout(True))

fmodel.run()

# Fails before the bugfix
print(fmodel.get_turbine_layout())
print(fmodel.get_turbine_layout(True))

With the bugfix, this produces

(array([  0., 500.]), array([0., 0.]))
(array([  0., 500.]), array([0., 0.]), array([90., 90.]))
(array([  0., 500.]), array([0., 0.]))
(array([  0., 500.]), array([0., 0.]), array([90., 90.]))

as expected. Prior to the bug fix (on the develop branch), the output is:

(array([  0., 500.]), array([0., 0.]))
(array([  0., 500.]), array([0., 0.]), array([90., 90.]))
Traceback (most recent call last):
  File "/Users/msinner/floris3/examples/dis887.py", line 18, in <module>
    print(fmodel.get_turbine_layout())
  File "/Users/msinner/floris3/floris/floris_model.py", line 1511, in get_turbine_layout
    xcoords, ycoords, zcoords = self.core.farm.coordinates.T
  File "/Users/msinner/floris3/floris/core/farm.py", line 465, in coordinates
    np.array([x, y, z]) for x, y, z in zip(
TypeError: 'numpy.float64' object is not iterable

Thank you to @AJP-47 for identifying this bug in #887, and to @rafmudaf for helping track it down.