Clean up __cinit__ methods of matrices

[jdemeyer]

1. Generic stuff like processing the parent should be done by the base class (matrix0.Matrix in this case) instead of by each derived class individually. This means that we will require that the first argument of every matrix constructor is the parent. Because of #25511, this is currently true everywhere.

2. In some cases, __cinit__ calls __init__ of the base class. That is really backwards and should never be needed!

This is a requirement for #23719.

[tscrim]

New commits:

​90a0b03

Clean up __cinit__ methods of matrices

[git]

Branch pushed to git repo; I updated commit sha1. This was a forced push. New commits:

​4eae803	Making matrices use the new _echelon_in_place method.
​e2f0550	Specify that _echelon_in_place shall return the pivots
​3c4a06d	Enable _mul_long for matrices
​1eaed37	More stuff in the meataxe interface, and a meataxe helper function
​8a06c0f	Fix docstring formatting
​13da208	Clean up creating Matrix_gfpn_dense matrices
​cb52ee3	Mark one doctest optional
​c8fa7bb	Clean up __cinit__ methods of matrices

[tscrim]

LGTM.

[jdemeyer]

Does this also imply a positive review of the dependency #25511?

[tscrim]

Replying to jdemeyer:

Does this also imply a positive review of the dependency #25511?

Only in the sense that #25511 works as it is suppose to by the doctests and the code looks okay. Hmm...I guess I am saying that aren't I?