Remove use of const_cast with Options

[cmacmackin]

const_cast gets used on const refs to Options (and GuardedOptions) objects in the following functions:

• T get(const Options&, const std::string&)
• T getNoBoundary(const Options&, const std::string&)
• bool isSetFinal(const Options&, const std::string&)
• bool isSetFinal(const GuardedOptions&, const std::string&)
• bool isSetFinalNoBoundary(const Options&, const std::string&)
• bool isSetFinalNoBoundary(const GuardedOptions&, const std::string&)

Even though the (Guarded)Options arguments are are logically const, the functions are modifying the attributes to mark that they have been read. This requires the const_cast.

I really don't like this. It doesn't look like isSetFinal is called in any context with a const& anyway? I know this is already an issue with the existing function, but can we avoid the const_cast here?

Originally posted by @ZedThree in #421 (comment)

The introduction of access controls (#421) and automatic ordering of components (#428) might make this real-time checking redundant in future, in which case we'd be able to remove these attribute writes, but it will be best to battle-test these new features before removing anything.