Align Mixscape with Seurat’s implementation (#710)

* Docstring clarifications

* Fixed layer scaling for marker DEG computation

* Added test_method parameter to DEG method call

* Introduced de_layer parameter and docs explanations

* Removed separate method for mean and std calculation

* Added custom Gaussian Mixture Model, Take positive and negative DEGs as perturbation markers

* Added Gaussian Mixture Model tests

* Resolved remaining ToDos

* Removed scanpy import

* Fixed split_by definition in mixscape method

* Fixed split_by definition in lda

* PR Revisions