README.md: add a brief blurb about motivation
Add the initial version of the Conditions API

This package provides a general framework for ops that perform boolean
logic. The code was migrated from net.imglib2.ops.condition, and adapted
to the ImageJ OPS paradigm.
Merge branch 'conditions'
Fix compile errors in Conditions API

The AbstractCondition class now extends AbstractOutputFunction, and
knows how to create its own outputs.

One unfortunate thing in the design right now is that the output is
typed on Boolean, which is immutable. So we actually need to compute the
value of the boolean in the createOutput method, rather than in the
safeCompute method as normal.

We may want to change this design before 1.0.0, to use a boolean-style
object whose value is mutable (e.g., BitType). Otherwise, the behavior
of the Conditions API is rather unintuitive, especially if a
preallocated Boolean is passed to the op, since its value cannot change.