bug: STLTMemory uses model.steps which does not exist in Mesa 4.x

st_lt_memory.py uses self.agent.model.steps which was removed in Mesa 4.x. Causes AttributeError during memory consolidation. 
The fix int(getattr(self.agent.model, "_time", 0)) is already applied in record_model.py 
this brings st_lt_memory.py in line. Fixed in PR #199