Add support for convolutional layers

[milancurcic]

This issue tracks the progress on the support for convolutional layers. Ideally will be using GH Projects for this but that's on an Org level; I don't want to pollute the modern-fortran Org with it, so when we transfer to github.com/neural-fortran Org, will set up some projects.

• [x] #58
• [x] #60
• [x] #63
• [x] #68
• [x] #61
• [x] #69
• [x] #62