proposed fix for #229

[selipot]

fix #229

[selipot]

the tests on the time axis fail which is obviously related to what this PR is intending on fixing. My issue is that I don't fully understand these tests. However, I think that my proposed fix can work, e.g.:

x = np.random.rand(10,20,100)
y = np.random.rand(10,20,100)
t = np.tile(np.arange(0,100),(10,20,1))
u,v = analysis.position_from_velocity(x,y,t,0,0,time_axis=0)
display(np.shape(x))
display(np.shape(u))

(10, 20, 100)
(10, 20, 100)

x = np.random.rand(10,20,100)
y = np.random.rand(10,20,100)
t = np.tile(np.arange(0,100),(10,20,1))
u,v = analysis.position_from_velocity(x,y,t,0,0,time_axis=1)
display(np.shape(x))
display(np.shape(u))

(10, 20, 100)
(10, 20, 100)

x = np.random.rand(10,20,100)
y = np.random.rand(10,20,100)
t = np.tile(np.arange(0,100),(10,20,1))
u,v = analysis.position_from_velocity(x,y,t,0,0,time_axis=-1)
display(np.shape(x))
display(np.shape(u))

(10, 20, 100)
(10, 20, 100)

[selipot]

replaced by #232