Refactor probnum.linalg to resolve the pylint messages

[nathanaelbosch]

At the time of writing, probnum.linalg triggers the following pylint messages:

• attribute-defined-outside-init
• too-many-statements
• too-many-instance-attributes
• too-complex
• protected-access
• too-many-lines
• no-self-use
• too-many-locals
• redefined-builtin
• arguments-differ
• abstract-method
• too-many-arguments
• too-many-branches
• duplicate-code
• unused-argument
• fixme
• missing-module-docstring

Goal:
Our long-term goal is to enable all of these checks for all parts of the project. Therefore, we should continuously work on fixing these. Currently, pylint is set up to have different checks for different modules, such that we can work on these problems individually and progress more quickly :)

To work on this issue you can check for these messages individually by calling:

pylint src/probnum/linalg --disable=all --enable=<message>

Then, after fixing all errors/warnings, you should remove the exception from the corresponding line in ./tox.ini, such that this message will be tracked in the future.

[marvinpfoertner]

Superseded by #523.