Add proto walking uptake function

[Robinlovelace]

uptake_walk_school_baseline = function(distance, upper_limit = 0.7, exponent = 1.3) {
  p = exp(-distance * exponent)
  p[p > upper_limit] = upper_limit
  p
}
# And...
uptake_walk_school_goactive = function(distance, upper_limit = 0.9, exponent = 0.8) {
  uptake_walk_school_baseline(distance, upper_limit, exponent)
}

[dabreegster]

f333f43 added this, I'll keep this issue open until I rewrite the docs on uptake configs

[dabreegster]

Example use in config.json:

"uptake": {
  "WalkToSchool": {
     "upper_limit": 0.7,
     "exponent": 1.3
  }
}

[Robinlovelace]

Distance in m.

Will install and test!