时间序列函数resamlpe详解

resample与groupby的区别：
resample：在给定的时间单位内重取样
groupby：对给定的数据条目进行统计

函数原型：
DataFrame.resample(rule, how=None, axis=0, fill_method=None, closed=None, label=None, convention='start', kind=None, loffset=None, limit=None, base=0)
其中，参数how已经废弃了。

rule：

常用的几个：A year
M month
W week
D day
H hour
T minute
S second

全部的：

B　　 business day frequency

C　    custom business day frequency (experimental)

D　　 calendar day frequency

W　　weekly frequency

M　　 month end frequency

BM　  business month end frequency

CBM　custom business month end frequency

MS　   month start frequency

BMS　 business month start frequency

CBMS　 custom business month start frequency

Q　　   quarter end frequency

BQ 　　business quarter endfrequency

QS　　 quarter start frequency

BQS 　  business quarter start frequency

A year   end frequency

BA         business year end frequency

AS 　　year start frequency

BAS       business year start frequency

BH 　　business hour frequency

H 　　   hourly frequency

T 　　   minutely frequency

S 　　   secondly frequency

L　　　 milliseonds

U 　　   microseconds

N　　    nanoseconds