Compiler integration of the Concat operation

[mdanilow]

This PR depends on a PR from the finn-hlslib where the Concat operation was implemented (Xilinx/finn-hlslib#143)

• The Concat op code is not hardcoded in the compiler anymore and it now accepts different datatypes of inputs. The output datatype is inferred.
• HLS implementation is defined for a one clock cycle, without defining NUM_REPS, so the new timeout cppsim template is also added and utilized by the operation.
• The tests/fpgadataflow/test_fpgadataflow_concat.py test now tests the case of inputs being different datatypes