BMP280 sensor (i2c) and c library
The struct bmp280_dev is defined in bmp280_defs.h
The actual memebers are 

bmp280_com_fptr_t read;
bmp280_com_fptr_t write;
bmp280_delay_fptr_t delay_ms;


If you search further in that file for  the keyword 


  bmp280_com_fptr_t


you will find this declaration

typedef int8_t (*bmp280_com_fptr_t)(uint8_t dev_id, uint8_t reg_addr,
    uint8_t *data, uint16_t len);


which gives that the functions you asking for are function-pointers.
Here is a nice tutorial about function pointers.
Programs as Data: Function Pointers

furthermore, it is essential to create theese functions and function-pointers, and to initialize a default return-value of rslt=BMP280_OK.
Here is a shorted example from my own code. You have to implant routines to get proper error-checking-

int8_t user_i2c_read(uint8_t dev_id, uint8_t reg_addr, 
uint8_t*reg_data, uint16_t len) 
{
  printf("\nuser_i2c_read\n"); //just to see that the function executes
  int8_t rslt = BMP280_OK;
   // implent more code here, to a start you can print out the
   // variables.and check that they are are 'set'
   if (rslt != BMP280_OK)
   {
      rslt = BMP280_E_COMM_FAIL; 
   }
   return rslt;
}


To the like for user_i2c_write 
   and 

void user_delay_ms(uint32_t ms){
    usleep(ms);
   }

Thoose functions are called from bmp280.c .
Also - when you from main run 

rslt = bmp280_init(&amp;bmp);   

Without creating thoose functions, the bmp280_init will never give you the right return values. 

Also it is worth mention that you do not need to name the functions to user_blabla. You can name them to whaatever you want - as long that you imitalize the functionpointers to the same name. That gives :you can re-initialize the function-pointrs to other names, and therefore make the read-call, the write-call etcetera to call other functions than they did previousley. 

I might have some stuff in this to not be 100% accurate, because I just a few hours ago, after working many hours with this, found it out out myself, and I have been up all to many hours to get this working. And I am not naturally english speaking, so there we have the languge-barriar.

One last hint, the issues thread from the repo of BoschSensortec/BMP280_driver has some code worth looking at. 
 BoschSensortec/BMP280_driver issuses