Required fields in our input files

[joshessman-llnl]

Continuation of earlier discussion on 3/17/21:

Consider the following C++ struct and its corresponding Inlet definition:

struct Foo
{
  bool bar = true; // has default
  bool baz; // no default
};

auto& foo_container = inlet.addStruct("foo"); // not marked as required - the struct as a whole is optional
foo_container.addBool("bar").defaultValue(true); // not required, but has a default
foo_container.addBool("baz").required(); // required, no default

In my opinion, the following should fail verification:

foo = {
  bar = true
}

Even though this matches the information already present in Inlet's internal representation, the explicit specification of a foo without including the required baz is what causes verification to fail.

This motivated LLNL/axom#495 as the Inlet's internal representation is not changed* by the reading of the above Lua input. The user's explicit specification of the foo table is what triggers the failure - if no input was provided, verification would pass even though neither the empty input nor the above input change Inlet's internal state.