get_sxdm_frame_sum() detector name check

See merge request !12

get_sxdm_frame_sum() now checks first whether the name of the detector defined as an argument matches with the .h5 files, if not, it raises and handles the error.

Update sxdm/process/math.py

See merge request !11

standart errors for cx, cy, cz

See merge request !10