GasPvtMultiplexer: Replace unholy trinity with visitor overload sets

[akva2]

Sits on top of #3278
Waiting for #3246

I realize this may be controversial, and that it might have runtime implications (which we need to benchmark) but;
This replaces what I consider the unholy trinity (void pointers, macros and SFINAE) with the visitor overload set idiom in the gas multiplexer.

As a bonus we avoid the need to have explicit ctors, copy ctors, assignment and comparison operators.

[bska]

As a bonus we avoid the need to have explicit ctors, copy ctors, assignment and comparison operators.

🎉

[akva2]

jenkins build this opm-models=767 opm-simulators=4325 please

[akva2]

benchmark please opm-models=767 opm-simulators=4325

[ytelses]

Benchmark result overview:

Test	Configuration	Relative
opm-git	OPM Benchmark: drogon - Threads: 1	1.026
opm-git	OPM Benchmark: drogon - Threads: 8	0.989
opm-git	OPM Benchmark: smeaheia - Threads: 1	1.02
opm-git	OPM Benchmark: smeaheia - Threads: 8	0.906
opm-git	OPM Benchmark: flow_mpi_extra - Threads: 1	1.001
opm-git	OPM Benchmark: flow_mpi_extra - Threads: 8	0.901
opm-git	OPM Benchmark: flow_mpi_norne - Threads: 1	0.995
opm-git	OPM Benchmark: flow_mpi_norne - Threads: 8	0.983

• Speed-up = Total time master / Total time pull request. Above 1.0 is an improvement. *

View result details @ https://www.ytelses.com/opm/?page=result&id=1899

[akva2]

benchmark please opm-models=767 opm-simulators=4325

[ytelses]

Benchmark result overview:

Test	Configuration	Relative
opm-git	OPM Benchmark: drogon - Threads: 1	0.989
opm-git	OPM Benchmark: drogon - Threads: 8	0.982
opm-git	OPM Benchmark: smeaheia - Threads: 1	1.006
opm-git	OPM Benchmark: smeaheia - Threads: 8	0.992
opm-git	OPM Benchmark: flow_mpi_extra - Threads: 1	0.987
opm-git	OPM Benchmark: flow_mpi_extra - Threads: 8	0.983
opm-git	OPM Benchmark: flow_mpi_norne - Threads: 1	0.989
opm-git	OPM Benchmark: flow_mpi_norne - Threads: 8	0.976

• Speed-up = Total time master / Total time pull request. Above 1.0 is an improvement. *

View result details @ https://www.ytelses.com/opm/?page=result&id=1912

[akva2]

I did my own benchmarking, and sadly it seems the variant approach is significantly slower (this is with the whole PR chain);

Total time (10 samples) for NORNE_ATW2013

procs	orig	variant
1	520.49 +- 3.53	551.50 +- 1.57 (p=0.00)
8	90.77 +- 0.34	99.45 +- 0.44 (p=0.00)

[bska]

sadly it seems the variant approach is significantly slower

That's disappointing.