main: Main function
In lixiao0214/GA: Genetic Algorithms package for STAT243 course

Returns the optimal x variable and every generation's AIC values

1 2	main(data, method, p, mutation_rate, regression_target, scheme, max_iter = 100)

`data`	data.frame: regression_target and covariates
`method`	the method to select the parents, "twopropselection", "onepropselection" or "tournament"
`p`	integer: splits number in crossover
`mutation_rate`	numeric: the rate of mutation in population
`regression_target`	character: Y variable
`scheme`	method of replacing population, "proportion" or "re-rank"
`max_iter`	the number of iterations given by the user