Decoupled and runtime-writable thresholds (#244)

* [CustomOp] groundwork for decoupled Thresholding op

* [Thresholding] more decoupled mode preparation

- new attributes for "weight" (really threshold) datatype + n_steps
- move thres datatype into acc minimization function and call in convert_to_hls
- other helper functions

* [Refactor] add make_weight_file for Thresholding layer + wire up

* [Threshold] first attempt at Thresholding_Stream_Batch

* [Threshold] wire up more decoupled functionality, add test

test does not yet pass for multi-PE

* [Threshold] use 2 layers of slicers, all decoupled cppsim passes now

* [Threshold] more decoupled fixes, most rtlsim tests pass

* [Threshold] add unroll to decoupled thresholding, remove unused vars

* [ConvertToHLS] allow specifying mem_mode in InferThresholdingLayer

* [Threshold] expose interfaces for runtime weight writing

* [Test] add runtime threshold test

not yet passing

* [Threshold] use std::less_equal also for decoupled

* [Threshold] use flipped PE axis for decoupled-more rtl thresholds

* [Test] runtime threshold reading works

* [Test] runtime writable thresholds test working

* [ConvertToHLS] fix threshold shape