Functional analog of hexagdly.Conv2d

[vaxenburg]

My research work requires a functional analog of the hexagdly.Conv2d class so I can use arbitrary parametrizations of the convolving kernels, something I couldn't do with the class. Similarly in spirit to torch.nn.functional.conv2d being a functional analog of torch.nn.Conv2d, I've implemented a functional conv2d version by isolating the functional part from the hexagdly.Conv2d class, modifying it slightly, and adding a wrapper. I thought it could be useful to others as well. Please consider pulling. Thanks for your great work!