[FileFormats.MOF] add support for MOF@1.7

[odow]

x-ref jump-dev/MathOptFormat#35

At the expense of type-stability, this makes things a lot simpler. But the rest of the nonlinear work showed that this shouldn't matter. And any other implementation is going to have to deal with the type ambiguity of variable/real/operator anyway.

Basic

• The file at src/FileFormats/MOF/mof.schema.json is updated
• The constant _SUPPORTED_VERSIONS is updated in
  src/FileFormats/MOF/MOF.jl

New sets

• New sets are added to the @model in src/FileFormats/MOF/MOF.jl
• New sets are added to the @enum in src/FileFormats/MOF/read.jl
• set_to_moi is defined for each set in src/FileFormats/MOF/read.jl
• head_name is defined for each set in src/FileFormats/MOF/write.jl
• A new unit test calling _test_model_equality is aded to
  test/FileFormats/MOF/MOF.jl

Tests

• The version field in test/FileFormats/MOF/nlp.mof.json is updated

Documentation

• The version fields are updated in docs/src/submodules/FileFormats/overview.md