Provide Iterator implementations for analyses

[johannesduesing]

This PR introduces Iterator implementations that correspond to the current MavenCentralArtifactAnalysis and MavenCentralLibraryAnalysis, respectively. The idea is to provide the same kind of data without forcing users to extend any specific base class.

Specifically, this PR adds:

• ArtifactAnalysisConfigBuilder and LibraryAnalysisConfigBuilder classes to programmatically build configuration objects. This is necessary as iterators will not necessarily be used in main methods, and we therefore need a way to build configurations without CLI argument parsing, see Rework configuration system #42

• MavenCentralArtifactIterator as an equivalent to the MavenCentralArtifactAnalysis. This iterator produces Artifact objects and supports all configuration options except multithreading.

• MavenCentralLibraryIterator as an equivalent to the MavenCentralLibraryAnalysis. This iterator produces LibraryResolutionContext objects to encapsulate all releases of a library. It supports all configuration options except multithreading.

• Custom input files are, for all analysis modes, now being validated syntactically before being processed. This ensures that only lists with valid G:A:V triples or G:A tuples, respectively, are processed any further.

With these changes, the following is now possible:

ArtifactAnalysisConfig config = new ArtifactAnalysisConfigBuilder()
                .withInputList(gavInputList)
                .withSkip(2)
                .withTake(5)
                .build();

MavenCentralArtifactIterator iterator = new MavenCentralArtifactIterator(true, true, false, config);

while(iterator.hasNext()){
    Artifact current = iterator.next();
    // Process artifact
}

What's left to do

• We need to update the README to reflect this new usage scenario
• We want to have multithreaded implementations - Sources are likely the best concept to do this. Should probably be a separate PR, but i am open to discussion

Closes #41 (for now, see follow-up #44)
Closes #42

[johannesduesing]

This PR will only deal with single threaded iterators. Multithreaded sources will be added as part of #44 .