Add Float16 support?

[milankl]

Running the following on an A64FX with native Float16 support yields

julia> @count_ops run_model(Float32,Ndays=5)
Starting ShallowWaters on Wed, 14 Apr 2021 05:36:36 without output.
60% Integration done in 1min, 22s.
Flop Counter: 221063593 flop
┌────────┬──────────┬─────────┐
│        │  Float32 │ Float64 │
├────────┼──────────┼─────────┤
│ muladd │        0 │  333040 │
│    add │ 91734565 │   72271 │
│    sub │ 22795413 │  166263 │
│    mul │ 97619024 │  244707 │
│    div │  2580653 │   81624 │
│    rem │        0 │      48 │
│    abs │        0 │   59320 │
│    neg │  4990000 │   48775 │
│   sqrt │        0 │    4850 │
└────────┴──────────┴─────────┘

julia> @count_ops run_model(Float16,Ndays=5)
Starting ShallowWaters on Wed, 14 Apr 2021 05:50:17 without output.
60% Integration done in 1min, 23s.
Flop Counter: 6919066 flop
┌────────┬─────────┬─────────┐
│        │ Float32 │ Float64 │
├────────┼─────────┼─────────┤
│ muladd │       0 │  333040 │
│    add │       0 │   72271 │
│    sub │       0 │  166263 │
│    mul │ 5575128 │  244707 │
│    div │       0 │   81624 │
│    rem │       0 │      48 │
│    abs │       0 │   59320 │
│    neg │       0 │   48775 │
│   sqrt │       0 │    4850 │
└────────┴─────────┴─────────┘

The second executes every Float32 in Float16, but there are not counted. I'd be happy to test this on A64FX if there's interest to include Float16 support.

[milankl]

Would it be enough to add (Float16, :16) here?

GFlops.jl/src/overdub.jl

Lines 26 to 29 in 03f2429

	const typs = (
	(Float32, :32),
	(Float64, :64),
	)

[milankl]

That seems to work on A64FX (UPDATE: x86 via LLVM's half yields identical results)

julia> @count_ops run_model(Float16,Ndays=5,scale=64)
Starting ShallowWaters on Wed, 14 Apr 2021 06:57:44 without output.
60% Integration done in 1min, 24s.
Flop Counter: 221063563 flop
┌────────┬──────────┬─────────┬─────────┐
│        │  Float16 │ Float32 │ Float64 │
├────────┼──────────┼─────────┼─────────┤
│ muladd │        0 │       0 │  333040 │
│    add │ 91734565 │       0 │   72271 │
│    sub │ 22795383 │       0 │  166263 │
│    mul │ 92043896 │ 5575128 │  244707 │
│    div │  2580653 │       0 │   81624 │
│    rem │        0 │       0 │      48 │
│    abs │        0 │       0 │   59320 │
│    neg │  4990000 │       0 │   48775 │
│   sqrt │        0 │       0 │    4850 │
└────────┴──────────┴─────────┴─────────┘

PR coming!