Use wider accum_t for (average) pooling

[vloncar]

Description

As discussed in 656 io_stream implementation of AveragePooling doesn't use the wider type for summing up the values of the pool window. This fixes it (bugs 1 and 2 from that discussion).

I'd appreciate an expedited review so that we can merge #656 (the test in main branch will fail if we merge that one before this one).

Type of change

• [x] Bug fix (non-breaking change that fixes an issue)

Tests

I expanded the test_cnn_mnist.py to include io_stream implementations where this issue was first observed.

Checklist

• [x] I have read the guidelines for contributing.
• [x] I have commented my code, particularly in hard-to-understand areas.
• [x] My changes generate no new warnings.
• [x] I have added tests that prove my fix is effective or that my feature works.