support exponential correlation structure for the `abess` python library

[Mamba413]

abess would be even more helpful if make_glm_data and make_multivariate_glm_data can support simulating datasets associated with an exponential correlation structure. This structure is widely considered in recent literature (e.g. https://arxiv.org/pdf/2104.12576.pdf).

[oooo26]

This is available now by giving an argument corr_type="exp" and I will close the issue. Thank you.