v0.7.3 for StatsPlots.jl; Draft installGuide

- We need to use v0.7.3 for StatsPlots.jl
- Probably also should confirm that the correct version of POMDPs is installed by build.jl
# Here is a draft install guide
```julia
Pkg.clone("https://github.com/sisl/Multilane.jl")

# Assuming we fixed the above in build.jl, the next step
# should be enough
Pkg.build("Multilane")

# Test
using Multilane
```
