Make RawParameter a String

[odow]

Lots of solvers mixed between Symbols and Strings. However, JuMP assumes that it is a String to use set_optimizer_attribute. We could even rename RawParameter to RawOptimizerAttribute?

MathOptInterface.jl/src/attributes.jl

Lines 664 to 672 in 2e99e25

	"""
	RawParameter(name)
	An optimizer attribute for the solver-specific parameter identified by `name`
	which is typically an `Enum` or a `String`.
	"""
	struct RawParameter <: AbstractOptimizerAttribute
	name::Any
	end