Implement generalized eigenvalue solver for general square matrices (LAPACK's ?ggev)

[bangconghuynh]

Implemented EigGeneralized which calls LAPACK's ?ggev for solving the generalised eigenvalue problem $AC = \lambda BC$ for general square matrices $A$ and $B$.

This PR closes#267 and is a more up-to-date version of #280.

[codecov]

Codecov Report

❌ Patch coverage is 96.01594% with 10 lines in your changes missing coverage. Please review.
✅ Project coverage is 87.44%. Comparing base (c7ff9dc) to head (3b555d5).
⚠️ Report is 2 commits behind head on master.

Files with missing lines	Patch %	Lines
lax/src/eig_generalized.rs	96.55%	8 Missing ⚠️
ndarray-linalg/src/eig.rs	81.81%	2 Missing ⚠️

Additional details and impacted files

@@ Coverage Diff @@## master #403 +/- ##
==========================================
+ Coverage 86.92% 87.44% +0.51% 
==========================================
Files 64 65 +1 Lines 3649 3895 +246 ==========================================
+ Hits 3172 3406 +234 - Misses 477 489 +12 

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