More R-like entrypoint

[jakobbossek]

Many users prefer the optim interface for sure. It should be possible to call ecr in te following way:

fn = function(x) sum(x^2)
res = ecr(fn, lower = c(-5, -10), upper = c(5, 10), representation = "float", n.population = 50L, ...)

[jakobbossek]

Added ecr function