Add `HighsLpRelaxation::Status::kTimeLimit`

[jajhall]

Firstly, the time limit set in HighsLpRelaxation::run needs to be non-negative, otherwise it's rejected when passed to lpsolver.setOptionValue("time_limit"

Secondly, if lpsolver.run(); results in a time-out, it currently returns HighsLpRelaxation::Status::kError so the time-out is ignored, and actions such as separation are performed.

Also, why do so many LP solves with bell5 have no basis?

[jajhall]

Actually needs

enum class Status {
kNotSet,
kOptimal,
kInfeasible,
kUnscaledDualFeasible,
kUnscaledPrimalFeasible,
kUnscaledInfeasible,
kUnbounded,
kTimeLimit,
kInterrupt,
kMemoryLimit,
kError
};

feeding into

enum class HighsMipInstruction {
kContinue,
kTimeLimit,
kInterrupt,
kMemoryLimit
};

where default behaviour follows from HighsMipInstruction::kContinue, otherwise flagged "bale out" takes place