FRLM get_vmt_coverage calls missing flow coverage method

[samay2504]

Description

FRLM.get_vmt_coverage() calls a non-existent method name (calculate_flow_coverage) when flow coverage data has not been prepared, which raises an AttributeError instead of computing coverage.

Steps to reproduce

# after creating an FRLM instance with solved flow data
if hasattr(model, "flow_coverage"):
    delattr(model, "flow_coverage")
model.get_vmt_coverage()

Expected behaviour

get_vmt_coverage() should initialise flow coverage using the existing public method and return VMT metrics.

Actual behaviour

An AttributeError is raised because calculate_flow_coverage is not defined on FRLM.

Environment

• spopt version: main
• Python version: 3.11
• OS: Windows 11