Convolution in 1D and 2D using fft

## Description
Functions to calculate convolution in 1D and 2D (circular and linear) using the FFT function in Stan. For instance,  `conv1(a,b)` and  `conv2(a,b)`. A general description of the algorithm is given [here](https://www.matecdev.com/posts/julia-fft-convolution.html#do-you-need-help-with-julia). 
