在每组中选择最大的N个组，使用熊猫。

My df:

我的df:

{'city1': {0: 'Chicago',
  1: 'Chicago',
  2: 'Chicago',
  3: 'Chicago',
  4: 'Miami',
  5: 'Houston',
  6: 'Austin'},
 'city2': {0: 'Toronto',
  1: 'Detroit',
  2: 'St.Louis',
  3: 'Miami',
  4: 'Dallas',
  5: 'Dallas',
  6: 'Dallas'},
 'p234_r_c': {0: 5.0, 1: 4.0, 2: 2.0, 3: 0.5, 4: 1.0, 5: 4.0, 6: 3.0},
 'plant1_type': {0: 'COMBCYCL',
  1: 'COMBCYCL',
  2: 'NUKE',
  3: 'COAL',
  4: 'NUKE',
  5: 'COMBCYCL',
  6: 'COAL'},
 'plant2_type': {0: 'COAL',
  1: 'COAL',
  2: 'COMBCYCL',
  3: 'COMBCYCL',
  4: 'COAL',
  5: 'NUKE',
  6: 'NUKE'}}
{'city1': {0: 'Chicago',
  1: