Type casting problems with SRTM DEM

[mbjoseph]

I was hoping to compute slope and aspect for some digital elevation models from the NASA SRTM mission, and ran into a type error that seems to be related to the _find_flats_edges function.

Here's a reproducible example Jupyter notebook that demonstrates the problem: https://gist.github.com/mbjoseph/9934454330b5715db9529502cf415f72#file-pydem-srtm-example-ipynb

A TypeError is raised:

TypeError: Cannot cast ufunc add output from dtype('float64') to dtype('bool') with casting rule 'same_kind'

It seems like similar issues have arisen elsewhere, e.g., numpy/numpy#7225

[mpu-creare]

@mbjoseph I'll have a look later today. I'd rather not intentionally cast unsafely, I'll see if there's another fix.

Thanks for finding this! I haven't kept up with changes in numpy -- evidently. Are you up and running at least? Are you on a windows machine?

[mpu-creare]

@mbjoseph could you try the proposed fix in 311d217 ? It's on the develop branch.

[mbjoseph]

Your fix with 311d217 worked beautifully, thanks! Also I was able to replicate this on OSX and Ubuntu - I haven't tried on a Windows machine.

[mpu-creare]

Excellent, I'll push it into master for the next release.