Begin LAMBDA algorithm
[GNSS] lambda_fix: Input checks
[GNSS] lambda_ldl: Function created and done.
[GNSS] nav: fix docstring, begin ldl_decorrel function
[GNSS/nav]: lambda_ztran - ZTRAN algorithm on Sec 3.4 on LAMBDA implementation docs

mu is negative here instead of positive like in pseudocode, to mimic how alpha
is selected in the text decription.