Refactor highspy for enhanced usability and performance

[mathgeekcoder]

This commit significantly improves the Highs class within highs.py, focusing on enhancing usability, efficiency, and robustness. Key changes include:

• Added comprehensive docstrings.
• Improved methods for adding, deleting, and retrieving multiple variables and constraints, for a more flexible and efficient API.
• Standardized some API conventions. Note, this includes a minor breaking change for the constraint value/dual methods.
• Updated tests and examples.

• Removed highs_batch. Order(s) of magnitude performance improvements. E.g.,

perf_old = [h.addVariable() for _ in range(100000)]   # 307.96 seconds

perf_new = [h.addVariable() for _ in range(100000)]   #   0.90 seconds
perf_new = h.addVariables(100000)                     #   0.04 seconds