Cpp metrics plugin

[intjftw]

This is the basis of the C++ metrics plugin. It contains the usual components of a plugin, including the fundamental db tables to store module-level and AST node-level (function, class) metrics. This plugin is intended to help Software Technology lab students start their projects.

[mcserep]

Made a quick initial review based on the code.

[mcserep]

Looks good with the changes 👏

[mcserep]

Incremental parsing should be supported by the new plugin as well.
The workflow for incremental parsing is the following (in case of an initial or forced parsing, only step 5 is executed):

1. directly modified files are detected by ParserContext.
2. all plugin parsers mark the indirectly modified files.
3. all plugin parsers perform a cleanup operation.
4. global tables are cleaned up by parser/src/parser.cpp.
5. all plugin parsers perform a parsing operation.

Possible solutions:

• For simple metrics without external relation, like the function argument count metric, a simple cleanup could be performed to remove all entries from the database, which belongs to modified (or deleted) files. Then only these files needs to be reparsed.
• Alternatively, a complete cleanup of all C++ metrics could be performed, not supporting incremental parsing for this plugin initially, but instead enforcing a complete recalculation of all metrics.

[mcserep]

In the loop above you are already iterating through all files and purging the ones, which has deleted, modified or action changed status. There is no need for another loop or for the _astNodeIdCache at all, we could simply purge the AST nodes there as well.

[mcserep]

I will accept it like this, so we can merge this, and created #670.