[BugFix] Subclass Construction Locpot<:VolumetricData

[jmmshn]

Subclass Construction Locpot<:VolumetricData

The constructor signature for Locpot is different from the parent.
This makes it hard to write general code to handle different kinds of VolumetricData objects.

Just adding a **kwarg there should work.

[janosh]

at the risk of sounding pedantic, can we add a test for this?

[jmmshn]

No problem! The test I added passes with the new code but not the old code.

[janosh]

thanks @jmmshn!