RandomAccessible: fix up javadoc
FinalInterval: tweak javadoc
Axes: minor fixes and refactoring

The following changes are part of this commit:
  * Add ability to construct custom spatial axis types
  * Rename CustomAxisType to simpler CustomType (makes calling
    "new Axes.CustomType" directly less redundant-looking)
  * Tweaked javadoc
AxisType: tweak javadoc
TypedSpace: each AxisType can only be used once

We have generally assumed that for a single TypedSpace, there cannot be
multiple axes with the same type. Maybe later we can add argument
checking for it, but for now, let's just mention it in the javadoc.
WIP: CombinedSpace and CombinedRealInterval
WIP: interface bloat