Add walk as default value for first and last legs in PublicTransportMode

[Mind-the-Cap]

Add walk as default value for first and last legs in PublicTransportMode, as it will be easier for users

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 70.42%. Comparing base (c3cea11) to head (59acadb).
⚠️ Report is 10 commits behind head on main.

Additional details and impacted files

@@           Coverage Diff           @@
##             main     #245   +/-   ##
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  Coverage   70.42%   70.42%           
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  Files          56       56           
  Lines        2424     2424           
=======================================
  Hits         1707     1707           
  Misses        717      717           

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[FlxPo]

I see one potential problem : the parameters of the default WalkMode used in the PublicTransportMode will be different from the parameters used by the user configured "single mode" WalkMode.

Having different parameters could be a good idea to differentiate "single mode" walking from "multimodal" walking, but hiding this in defaults seems risky ?

The old behavior is :

walk = mobility.WalkMode(
    transport_zones,
    generalized_cost_parameters=mobility.GeneralizedCostParameters(
        cost_constant=10.0
    )
)

walk_intermodal_transfer = mobility.IntermodalTransfer(
    max_travel_time=20.0/60.0,
    average_speed=5.0,
    transfer_time=1.0
)

walk_pt = mobility.PublicTransportMode(
    transport_zones,
    first_leg_mode=walk,
    last_leg_mode=walk,
    first_intermodal_transfer=walk_intermodal_transfer,
    last_intermodal_transfer=walk_intermodal_transfer
)

modes = [walk, walk_pt]

which is not equivalent to the new behavior :

walk = mobility.WalkMode(
    transport_zones,
    generalized_cost_parameters=mobility.GeneralizedCostParameters(
        cost_constant=10.0
    )
)

walk_pt = mobility.PublicTransportMode(
    transport_zones
)

modes = [walk, walk_pt]

Because the walk_pt will now use default parameters.