saxs::Profile creates its own random number generators internally in functions

[drussel]

This is a bit problematic because

• the generators are not seeded randomly (so they generate the same sequence each time)
• it doesn't allow the seed to be controlled from the command line or otherwise

They should probably use base::random_number_generator instead.

[duhovka]

it is ok, since it is used to generate error bars if they are missing.