Variablethinmultipole WHITENOISE buffer

[oscarxblanco]

The PR adds the possibility to have a buffer for the WHITENOISE mode of the VariableThinMultipole.

Two new parameters are added: BufferSize and BufferOffset. The buffer size sets the lenght of the buffer and allows to store in an array the random value used in every turn, while the buffer offset sets the first turn to be stored in the array.

The parameter seed is removed because it had no use in this pass method. Instead, the seed for the random number generator should be set through the tracking functio, and it is mentioned in the help message of the element.

[oscarxblanco]

Dear all, who could review this PR ?

[swhite2401]

@oscarxblanco , isn't this already covered by FuncA, FuncB? -> the user can provide an arbitrary buffer

[oscarxblanco]

Hi @swhite2401 ,
I understand that this history buffer could be used to calculate linear optics parameters from gaussian noise. You mentioned it in this conversation
#839 (comment)
I will quote it here again,

... The other one is a bit more tricky, because you want to be able to access what is applied to the beam. However, I understand the complaints, we could instead think of adding a history buffer attribute that is filled during the tracking. This already exists for other elements like the beam monitor or beam loading for example, what do you think?
I you decide to go for this option you have to be very careful: the buffer needs to be properly initialized depending on the number of turns otherwise you will get segmentation faults

[swhite2401]

These are 2 different things, this comments was relating to the possible of saving a history of Polynom[A/B], to record was was applied to the beam turn by turn.

What I mention here is case 2 in the passmethod, it allow to load any buffer that would then be applied periodically to the beam, which I believe covers what you implemented here

[oscarxblanco]

@swhite2401 , ok, this is my mistake. Sorry.

[oscarxblanco]

@swhite2401 , I will close this PR. Thank you again, and sorry for the distraction.