425 add n (sample size) as output of pw_info

[LittleBeannie]

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[LittleBeannie]

Hi @jdblischak and @nanxstats , in this PR, I left some dplyr operations (see the last few lines of pw_info). I have not changed them into data.table because I have not figured out how to join 2 tables by columns with different names. For example, table 1 has columns of A, B, C, and table 2 has columns of AA, D, E. How can I left join table 1 with table 2 by A = AA?