Fix: handle MPS OBJSENSE MAXIMIZE in reader

[LucasBoTang]

Fixes #92.

Problem

Two bugs in src/mps_parser.c made every maximization MPS instance silently wrong (status OPTIMAL, no warning) — full write-up in #92.

1. Wrong optimum. The OBJSENSE value line (MAXIMIZE) is a single token, which the section detector consumed as a (bogus) header, so is_maximize was never set and the problem was solved as a minimization.
2. Wrong reported objective sign. The parser negates the objective to convert max→min, but the sense was a parser-local flag that was never propagated, so the reported objective kept the wrong sign.

Fix

• Treat a single token as a section header only when it matches a known section keyword; otherwise fall through to the section handler, so the OBJSENSE value reaches case SEC_OBJSENSE.
• Carry the sense on lp_problem_t (objective_sense_t objective_sense), derive it once into objective_sign on the solver state, and apply it when reporting the primal/dual objectives (and in the presolve postsolve path).

Verification

• maximize x s.t. x <= 5 → x = 5, objective = 5 (previously x = 0, obj = 0)
• a minimization instance is unchanged
• builds clean; both the main-solver and presolve paths report the correct sign

[ZedongPeng]

We might also need to support the same line section declaration.
https://github.com/scipopt/scip/blob/f8a5aa2ac5681bbbdf41be6e491f7d0c3e84299d/src/scip/reader_mps.c#L788-L799