Allow pycbc_multi_inspiral to search over a sky grid (#4380)

* Allow pycbc_multi_inspiral to search over a sky grid

* Adding the possibility to give either a sky grid or a sky position

* Change the names of the command lines --ra and --dec to --latitude and --longitude to match the changes made in pycbc_multi_inspiral and remove --processing-scheme because it makes the example crash

* Compute the antenna pattern only once for each sky position by keeping fp and fc together

* Replace the number of detectors from a hardcoded value to a variable