Sigmas math

[vmobilis]

@rmatif, hello, maybe add very basic calculations ability?
This would allow to shorten linear sequences.
For example, --sigmas 15,-0.5 wil make, for 5 steps, sigmas:
15.0, 14.5, 14.0, 13.5, 13.0, 0.0

Or --sigmas 16,^0.5,^0.5,^0.5,-1 will make, for 7 steps:
16.0, 4.0, 2.0, 1.41, 0.41, -0.59, -1.59, 0.0.

Last math operation is, first, remembered in last 2 positions of params.custom_sigmas and then is used to fill missing sigmas for whole steps range.