subfield_content: Extract specific subfield from funds dataframe
In Volodin-DD/rusmarcFunds: Work with rusmarc format easily.

Extract specific subfield from funds dataframe

1 2	subfield_content(df, field_name, subfield_name, preserve_field_names = TRUE, preserve_field_number = FALSE)

`df`	A dataframe made by read_funds_txt.
`field_name`	A string of field_name 000-999.
`subfield_name`	A string of subfield name starting with "$".
`preserve_field_names`	A logic. Whether field names should be saved in resulting dataframe. TRUE by default.
`preserve_field_number`	A logic. Whether field numbers should be saved in resulting dataframe. FALSE by default.

1 2	subfield_content(df = fund_df, field_name = "200", subfield_name = "$a") subfield_content(df = fund_df, field_name = "210", subfield_name = "$c", preserve_field_names = FALSE)