Sparse autocov matrix classes

[j-i-l]

Currently we have two of them:

https://github.com/alexbovet/flow_stability/blob/8648a8bd715e3b086ed13cb7453d41754c176398/src/flowstab/SparseStochMat.py#L1175-L1176

and

https://github.com/alexbovet/flow_stability/blob/8648a8bd715e3b086ed13cb7453d41754c176398/src/flowstab/SparseStochMat.py#L1454-L1455

With a much larger usage pattern for sparse_autocov_mat. Actually, sparse_autocov_csr_mat only occurs twice as potential substitute for `sparse_autocov_mat' here and here thus the called methods overlap.

I did not check every method in detail, but to me it seems that sparse_autocov_csr_mat could either be dropped completely, or inherit from sparse_autocov_mat and overwrite some key methods.

@alexbovet what is your thought on this?