DX conversion fails

[orbeckst]

Creating DX files for PMF grids fails because BP did not keep up with GridDataFormats.

Code needs to be changed to work with latest GDF.

Reported by @ChenouZhang

[orbeckst]

Commit 1019419 in PR #11 solves this problem. @ChenouZhang if you make a separate PR with the fix I'll merge it right away.

[ChenouZhang]

PR #13 fixes this problem.