* feat: corr_matrix kwargs as input for least squares fit
* feat/tests: inverse covariance matrix and correlation matrix kwargs as input for least squares function
* feat/tests/example: reduced new kwargs to 'inv_chol_cov_matrix' and outsourced the inversion & cholesky decomposition of the covariance matrix (function 'invert_corr_cov_cholesky(corr, covdiag)')
* tests: added tests for inv_chol_cov_matrix kwarg for the case of combined fits
* fix: renamed covdiag to inverrdiag needed for the cholesky decomposition and corrected its documentation
* examples: added an example of a correlated combined fit to the least_squares documentation
* feat/tests/fix(of typos): added function 'sort_corr()' (and a test of it) to sort correlation matrix according to a list of alphabetically sorted keys
* docs: added more elaborate documentation/example of sort_corr(), fixed typos in documentation of invert_corr_cov_cholesky()
