Create RelativeGapTolerance and AbsoluteGapTolerance optimizer attributes

[mtanneau]

Addresses #1936

I have created a RelativeGapTolerance attribute. If that's satisfactory code- and doc-wise, I'll do the same with AbsoluteGapTolerance.

TODO

• RelativeGapTolerance attribute
• AbsoluteGapTolerance attribute
• Docs update?

[mtanneau]

Is there anything I should change in the docs? Maybe around here?

MathOptInterface.jl/docs/src/reference/models.md

Lines 66 to 77 in 868aa29

	## Optimizer attributes
	```@docs
	AbstractOptimizerAttribute
	SolverName
	SolverVersion
	Silent
	TimeLimitSec
	RawOptimizerAttribute
	NumberOfThreads
	RawSolver
	```

[odow]

Yip. That's the place to add the docstrings.

You could also add it to:
https://github.com/jump-dev/MathOptInterface.jl/blob/master/docs/src/tutorials/implementing.md#implement-attributes

The attribute value type should be tested:

MathOptInterface.jl/test/attributes.jl

Line 225 in 868aa29

@test MOI.attribute_value_type(MOI.RelativeGap()) == Float64

It also needs a test for solvers implementing this. Something like:

MathOptInterface.jl/src/Test/test_attribute.jl

Lines 164 to 191 in 868aa29

	"""
	test_attribute_TimeLimitSec(model::MOI.AbstractOptimizer, config::Config)
	Test that the [`MOI.TimeLimitSec`](@ref) attribute is implemented for `model`.
	"""
	function test_attribute_TimeLimitSec(model::MOI.AbstractOptimizer, ::Config)
	@requires MOI.supports(model, MOI.TimeLimitSec())
	# Get the current value to restore it at the end of the test
	value = MOI.get(model, MOI.TimeLimitSec())
	MOI.set(model, MOI.TimeLimitSec(), 0.0)
	@test MOI.get(model, MOI.TimeLimitSec()) == 0.0
	MOI.set(model, MOI.TimeLimitSec(), 1.0)
	@test MOI.get(model, MOI.TimeLimitSec()) == 1.0
	MOI.set(model, MOI.TimeLimitSec(), value)
	@test value == MOI.get(model, MOI.TimeLimitSec()) # Equality should hold
	_test_attribute_value_type(model, MOI.TimeLimitSec())
	return
	end
	test_attribute_TimeLimitSec(::MOI.ModelLike, ::Config) = nothing
	function setup_test(
	::typeof(test_attribute_TimeLimitSec),
	model::MOIU.MockOptimizer,
	::Config,
	)
	MOI.set(model, MOI.TimeLimitSec(), nothing)
	return
	end