Make MvnFreeNPathMetric report NPath values for each method in each class of a file

[ivanovmg]

Currently MvnFreeNPathMetric computes a total NPath for all methods in a class.
However, in order to make it usable for real applications, it is necessary to make it format output in a more structured way.

Consider these options below.

Pure text format

- filename
  - Class1
    - method1 - 1
    - method2 - 5
    - method3 - 2
  - Class2
    - method1 - 2
    - method2 - 3
    - method3 - 1

Nested dictionary

>>> pprint(npath_metric.value())
{
   'Class1': {
    'method1': 1,
    'method2': 5,
    'method3': 2,
  },
   'Class2': {
    'method1': 2,
    'method2': 3,
    'method3': 1,
  },
}

Benefits of the nested dictionary format is that:

• it can be used to compute total NPath for a class, highest Npath for a class from a file, etc
• it can be easily re-formatted to a plain text later on, when necessary

However, we need to consider this intention to unify interfaces of Metric and Pattern #813. There it was suggested that the interfaces should produce int as an output, which contradicts this approach of splitting NPath for each method.

@yegor256 please have a look.
Does a nested dictionary format sound like a good idea?
What do you think of the expected output? If it is enforced to be an int, then do we need to produce a total NPath for a whole file? In my opinion, it does not make a clear picture of the metric - it can give high value for a class with many simple methods, which is not reflective of a bad code.

@kodsurfer there is quite some stuff to do.
From the top of my head, currently MvnFreeNPathMetric works fine when it comes to finding a total NPath for all methods in a class (https://github.com/cqfn/aibolit/blob/master/aibolit/metrics/npath/main.py#L93). So, when it comes to computing NPath, it works OK, but the resulting output is probably not what we ultimately expect.

It seems reasonable to provide the report on NPath metric for a given file in the following format, maybe:

- filename
  - Class1
    - method1 - 1
    - method2 - 5
    - method3 - 2
  - Class2
    - method1 - 2
    - method2 - 3
    - method3 - 1

@yegor256 what's your take on the output format?

Originally posted by @ivanovmg in #796