总结两行，保持多个变量的平均值

Question

假设我有以下数据：

df1 <- data.frame(Case_ID = c("CH1990", "CH1991", "CH1991", "GE1990", 
   "GE1991"), Year =rep(c("1990", "1991", "1991", "1990", "1990")),
    Country = rep(c("China", "Germany"), c(3, 2)),
    Continent = rep(c("Asia", "Europe"), c(3, 2)),
    GDP = c(6, 8, 5, 11, 0), Population = c(5,10,6,4,0), 
    stringsAsFactors = FALSE)

Case_ID Year Country Continent GDP Population
1  CH1990 1990   China      Asia   6          5
2  CH1991 1991   China      Asia   8         10
3  CH1991 1991   China      Asia   5          6
4  GE1990 1990 Germany    Europe  11          4
5  GE1991 1990 Germany    Europe   0          0

我想将具有相同Case_ID的行替换为显示变量GDP和人口平均值的一行，结果如下：

df2 <- data.frame(Case_ID = c("CH1990", "CH1991", "GE1990", "GE1991"),
              Year =rep(c("1990", "1991", "1990", "1990")),
              Country = rep(c("China", "Germany"), c(2, 2)),
              Continent = rep(c("Asia", "Europe"), c(2, 2)),
              GDP = c(6, 6.5, 11, 0), Population = c(5,8,4,0), 
              stringsAsFactors = FALSE)

Case_ID Year Country Continent  GDP Population
1  CH1990 1990   China      Asia  6.0          5
2  CH1991 1991   China      Asia  6.5          8
3  GE1990 1990 Germany    Europe 11.0          4
4  GE1991 1990 Germany    Europe  0.0          0

我目前计算两个变量的平均值，使用我的结果添加新行，然后删除其他行。有更短的方法吗？

谢谢！