CNN backward pass

[milancurcic]

This PR implements

• Backward pass for the conv2d layer (closes conv2d backward pass #61)
• Backward pass for the maxpool2d layer (closes Implement maxpool2d backward pass #69)
• Example of CNN training on MNIST (closes Add a MNIST training example using conv2d layers #62)
• Tests for the backward passes for maxpool2d and conv2d
• Tests should pass with ifort; they currently pass only with gfortran

Altogether closes #64.

[milancurcic]

Tests pass with ifort-2021.7 in release mode, but not in debug mode. Unclear yet why. Tests that used to pass with ifort-2021.6 in debug mode don't pass anymore.