plotextractor: initial release

• This is part one of the merge of Valkyrie's latest plot extractor branch. This initial commit contains only the plot extrator library itself. It is a work in progress, intended mostly for Jan to take over the feature development. (So CLI API may change etc.)
  • During merge I have fixed only the most important issues such is missing imports or non-initialized variables so that the basic use case of `plotextractor -d 2008' would work well. There are many important issues that remain to be fixed though, see the kwalitee report.
  • In addition, I have left several FIXMEs in the code, some are equally important such as generating proper 001/035 or using proper shell command escaping with timeouting mechanism to prevent image conversion processes from running away.

• The other parts of Valkyrie's plot extractor branch that deal with plugging the plot extractor into the workflow and with presenting the Plots tab to the user are independent of the plot extractor library and will be merged later.

• This commit addresses #153.