Add initialization logic to accumulation register

[norabarlow]

The previous accumulation register logic works great, except for the first time the register is used. The register value isn't initialized to anything, which messes things up.

To fix this, I

• added selector that is true when it is the first iteration
• move mux to in front of the adder that chooses between the bias and accum_reg. chooses bias on the first iteration; accum_reg otherwise

Note: this still outputs valid on the last iteration. We could change it to output and initialize on iteration 0 (would save a PE by removing a selector), but we would need a register to hold the in_valid signal of the previous sequence as we move to a new computation sequence. I chose not to do this for now but it can be changed later.