More specific type hierarchy for volatile types

[bogovicj]

Currently, VolatileIntType extends AbstractVolatileNativeRealType, and is therefore a RealType.
It is not currently an IntegerType, and probably should be.

The same is true for other Volatile types:

• VolatileByteType
• VolatileUnsignedByteType
• VolatileShortType
• VolatileLongType
• VolatileUnsignedLongType
• VolatileARGBType

Adding AbstractVolatileNativeIntegerType and having the appropriate volatile types extend that would be a good step, and would be similar to the type hierarchy for non-volatile types (see AbstractIntegerType for example).

Some specific motivation

This issue came up in n5-viewer
saalfeldlab/n5-viewer#34

one fix to that issue is with this PR in imglib2-ij
imglib/imglib2-ij#35

but this is the deeper issue. Some, maybe all of the changes in the above PR would not be necessary if the type hierarchy for volatile types were more fine-grained. e.g., if volatile types implemented IntegerType, the explicit checks using volatile types in the PR would not be necessary, because there is currently a check for IntegerType.