highspy: updated highs_linear_expression to be immutable for issue #1888

[mathgeekcoder]

Changes should fix #1888

• changed highs_linear_expression to be immutable by default
• added __repr__ for easier debugging and pretty print linear expressions
• added __iadd__ for mutable operations
• added qsum for faster aggregation
• added expr == [lb,ub] -> lb <= expr <= ub syntax
• updated chained comparison support to work in immutable setting

Note there are some low-risk caveats for the chained comparison support. I have documented them in the code. Though I believe the benefits outweigh the potential concerns.

[odow]

I can't comment on the implementation (I don't know Python well), other than to say that, at minimum, this needs a bunch of tests.

[mathgeekcoder]

I can't comment on the implementation (I don't know Python well), other than to say that, at minimum, this needs a bunch of tests.

Agreed. Working on this now, will update PR. BTW: the code changes for immutability are very simple. The chained comparison support definitely needs more testing, and I believe I've already found a couple of edge cases that need attention.

[mathgeekcoder]

Abandoning this PR, as it has been replaced by #1942.