Network metrics

[clhunsen]

As part of issue #11, we need to add an extensive list of networks metrics to the library for any further network analysis. The basic implementation can be found here already.

The next steps are:

• incorporate all comments from https://github.com/hechtlC/codeface-extraction-r/commit/3330ab5cff55fece4882b2ab896015c4e7943562,
• merge everything on https://github.com/hechtlC/codeface-extraction-r (branch master), and
• open a pull request from there.

Note: This issue is basically there for bookkeeping.

[bockthom]

As #78 is merged into the dev branch, this can be regarded as closed.