C source code for np.einsum

[dhjx1996]

Hi @riclarsson, this is an idea I had ever since we discussed the importance of numpy.einsum in making PythonicDISORT fast, sorry that I am following up on it only now (and maybe you have already done this).

You probably know that all of NumPy, SciPy not to mention many base Python functions are implemented in C. Here is the source code for np.einsum: https://github.com/numpy/numpy/blob/e2805398f9a63b825f4a2aab22e9f169ff65aae9/numpy/core/src/multiarray/einsum.c.src. Would such C source code help in porting important NumPy, SciPy and other Python functions into ARTS? A well-built einsum algorithm may also greatly speed up tensor operations in other parts of ARTS. In this vein, the C source code of the scipy.sparse library may be worth looking into as well.