anlzr: replace AverageComplexity field by a ComplexityMetric structure

A simple number is not enough for analyzing the complexity of a whole
project. Multiple metrics can be used and can be applied to different
part of a project (per file, per function, per class, etc.). Therefore,
a ComplexityMetric structure allows more flexibility for further
improvements and make a clear separation in the Result structure.
src: add a FIXME and TODO annotations

For the FIXME: In a function (or method), return statements may occur at
multiple locations (eg. into a If statement) and not only at the end of
the function. Therefore, it makes sense to take all of these statements
into account in the API.

For the TODO: Loop statement is more generic term than "For" or "While".
It makes more sense here to use the more abstract form. Moreover, we
also need to improve this Statement by adding the loop condition.