Tileset info not found with custom data

Hi,

I tried the example code from https://docs.higlass.io/jupyter.html#numpy-matrix.
```
import numpy as np

dim = 2000
I, J = np.indices((dim, dim))
data = (
-(J + 47) * np.sin(np.sqrt(np.abs(I / 2 + (J + 47))))
- I * np.sin(np.sqrt(np.abs(I - (J + 47))))
)

from clodius.tiles import npmatrix
from higlass.tilesets import Tileset

ts = Tileset(
tileset_info=lambda: npmatrix.tileset_info(data),
tiles=lambda tids: npmatrix.tiles_wrapper(data, tids)
)

display, server, viewconf = higlass.display([
View([
Track(track_type='top-axis', position='top'),
Track(track_type='left-axis', position='left'),
Track(track_type='heatmap',
position='center',
tileset=ts,
height=250,
options={ 'valueScaleMax': 0.5 }),

])
])
display
```
And I got this

And this is when I try `ts.tileset_info()`

Could you give me some suggestions on how to solve this?
Thank you!

Reproduce the NumPy matrix example from the issue and inspect the custom Tileset entry points, especially npmatrix.tileset_info(data), npmatrix.tiles_wrapper(data, tids), and ts.tileset_info(). Compare the returned tileset information with what the heatmap display expects; done means ts.tileset_info() succeeds and the example renders the custom data without the reported error.

Written by the indexing model from the issue text.