Therapi-based Discovery

[gselzer]

This PR adds a Therapi-based Op Discovery mechanism via the new scijava/scijava-discovery-therapi module. This module enables "element-tagged" discovery through the use of the @implNote javadoc tag.

How do I declare an Op using Therapi?

1. Enable therapi annotation processing in your POM.
2. Add an @implNote to any Class, Method or Field. The @implNote annotation should be followed with (1) the tag type (i.e. the keyword under which this AnnotatedElement will become discoverable) and (2) any (comma-delineated) key-value pairs that should be scraped with the tag.

Note that the tags are parsed using Parsington, as the org.scijava.parse package from SciJava Common was moved into the incubator and modularized into SciJava Parse2.

An Op written as a Field discoverable using therapi might look like:

/**
 * @implNote op names='foo', priority='100'
 */
public final Function<Integer, Integer> fooOp = (in) -> {...};

Ops written as Methods require a type option:

/**
 * @implNote op names='bar', type='java.util.function.Function'
 **/
public static Integer bar(Integer in) { ... }

This PR also refactors the Discoverer and Discovery API from SciJava Discovery to make the discovery returns much more powerful.

Furthermore, this PR also introduces SciJava Parse2 to the incubator, partitioning out org/scijava/parse from SciJava Common.

Closes scijava/scijava#77

[gselzer]

Note: This is the first step towards completing scijava/scijava#56

[ctrueden]

🚀