grid.collapse returns NaNs when granular temperature is initiated with grid containing values.

[dwerner95]

Following code returns a field of NaNs

import up4
import numpy as np

data = up4.Data("test.hdf5")
grid = up4.Grid(data)
velfield = data.velocityfield(grid)

gran_temp_field = data.granular_temperature(field)
gran_temp_2d= gran_temp_field.collapse(0)

print(np.isnan(gran_temp_2d).all())

## returns True

While it is not the way the grid should be used, this bug should not happen.
gran_temp_field contains numbers that are not NaN, the grid.collapse() function should be able to produce a proper field. (slices work)

[DanW97]

Aren't you meant to be on holiday?