MPL: honor max macro and max std_cell thresholds during merge of small children

[joaomai]

Description

The function ClusteringEngine::mergeChildrenBelowThresholds allows the merged clusters to have a higher amount of macros/cells than the values set in max_macro_ and max_std_cell.

Suggested Solution

Short circuit the attemptMerge call with a check for the amount of total macros and std_cells that the merged cluster will have.

Additional Context

No response