Generate Java call graphs from local JARs in OPALPlugin

[mir-am]

Description

As discussed in #363, it is now possible to generate call graphs from local JARs in the .m2 folder, created by POMAnalyzer.
This PR makes the following changes:

• Changes the DataWriter interface to DataRW so that the plugins can read/write from/to the disk given a base directory.
• OPALPlugin first tries to generate a call graph from a local JAR file if available. If not, it downloads the JAR file from Maven central like before.

Motivation and context

POMAnalyzer stores JAR files to the disk when resolving dependencies. Therefore, it is desirable to generate CGs from local JAR files and also reduce the network activity of OPALPlugin.

Testing

Tested with the DC.

[proksch]

I think that the solution is sub-optimal... a case distinction that should be taken on a high level is dragged down through the various levels of the API. I have proposed a different strategy in the individual comments.

[proksch]

Do not use exception handling for programming logic! Concrete proposal: There is no need to have two separate methods, one createPartialCG method should be enough, which just takes a path to the jar to be analyzed. The only difference is where the path comes from: either mavenCoord.getPath() exists or the .jar is downloaded and a path to the downloaded file is provided. As the jar should always exist locally in our setup, I would be fine with just throwing an error when the file is missing.

[proksch]

Please merge the two createPCG methods into one. There is NO need for a case distinction, both should do exactly the same...

[mir-am]

Addressed in 2c4d825

[mir-am]

@proksch, thanks for the code review and comments.
I have addressed your comments. I admit that this is not the best solution but it leverages our existing JARs in .m2 and reduces the network traffic. Also, it downloads JARs, if not present in .m2.