Generalize BISG function to handle custom surname / geographic input

[pssachdeva]

Currently, the package assumes that BISG will merge with WRU's surname2010 dataframe. This needs to be extended to allow the user to provide their own surname dataframe.

The following changes must be made:

• compute_p_r_cond_s: this function needs to accept a surname dataframe, which defaults to NONE. If NONE, then merge_surnames from wru is called. Otherwise, the merge occurs with the provided dataframe. This should also include a race_cols argument, defaulting to the standard in wru.
• compute_p_r_cond_s_g: this function needs to accept a surname dataframe, as described above.
• bisg: this function needs to accept a surname dataframe, as described above.