Allow for multiple travel cost columns

[rafapereirabr]

The package currently allows users to pass only one travel cost column. However, it would be good to have a more flexible approach that allow users to pass multiple travel cost columns.

---

Edit by Daniel:

How would that work for each measure we have implemented?

• cost_to_closest(): we wouldn't have a single minimum cost for each origin anymore. Instead, the result would be a pareto frontier of optimal costs. Output would need to include a column for each travel cost, specifying the costs at each pareto frontier edge.
• cumulative_cutoff(): apply two+ restrictions at the same time. Output would need to include a column for each travel cost, in which the specified cutoffs would be listed.
• cumulative_interval(): similar to above, but we would need to calculate all possible combinations of cutoffs inside the specified interval. Then we would have to summarize the accessibility estimates calculated from all these combinations. Output would need to include a column for each travel cost, in which the specified intervals would be listed.
• balancing_cost(): similar to cost_to_closest().
• gravity(): would need to be able to receive a list of decay functions, each would be simultaneously applied, each with its own cost. Output would need to include a column for the decay function used for each travel cost, in which the specified decay parameters would be listed.
• spatial_availability() and floating_catchment_area(): similar to gravity().

[dhersz]

3045a20: cumulative_cutoff() works with more than one travel cost