adds L-BFGS-B (L-BFGS Bounded)

[khizirsiddiqui]

• Implements L-BFGS for the bounded problem

Addresses #151

Couldn't use any existing L-BFGS code for the search direction and gradient norms are different for bounded optimization.

% ./ensmallen_tests "[LBFGSB]"                                                                                  
ensmallen version: 3.11.0 (Sunny Day)
armadillo version: 15.2.4 (Medium Roast Deluxe)
random seed: 0
Filters: [LBFGSB]
===============================================================================
All tests passed (21 assertions in 7 test cases)

Disclaimer: Used LLM code generation for test case writing.