Simplify ArbitraryMotion struct

[pvillacorta]

Change

mutable struct ArbitraryMotion{T<:Real,V<:AbstractVector{T}} <: MotionModel{T}
    period_durations::AbstractVector{T}
    dx::AbstractArray{T,2}
    dy::AbstractArray{T,2}
    dz::AbstractArray{T,2}
    ux::Vector{LinearInterpolator{T,V}}
    uy::Vector{LinearInterpolator{T,V}}
    uz::Vector{LinearInterpolator{T,V}}
end 

to:

 mutable struct ArbitraryMotion{T<:Real} <: MotionModel{T}
     period_durations::AbstractVector{T}
     dx::AbstractArray{T, 2}
     dy::AbstractArray{T, 2}
     dz::AbstractArray{T, 2}
 end

Originally posted by @cncastillo in #184 (comment)

[pvillacorta]

The most efficient solution should be deleting the times function or, at least, calculate the time nodes for ArbitraryMotion using ranges between 0 and 1 and then scaling them with the values of period_durations

Also consider periodic and non-periodic ArbitraryMotion.

• Periodic should start and end with dx = 0
• Non-periodic starts at 0 but does not need to end in 0

Also simplify adapt functions in GPUFunctions.jl

[pvillacorta]

Also, we need to re-implement resetmag matrix for flow dynamics, as it was removed from ArbitraryMotion for simplicity