Hardcoded min size reading raw transmissions

[armengau]

In raw_io.read_transmission_file , it is implicitely assumed that the input transmission files have num_pixels >= 50
It may break code for some specific mocks with short transmissions.

This is at two places:

picca/py/picca/raw_io.py

Line 118 in 5b92222

w = num_pixels >= 50

picca/py/picca/raw_io.py

Line 148 in 5b92222

if w.sum() < 50: