Fix a bunch of clang-tidy issues in Options

[ZedThree]

There's a small number left now:

include/bout/options.hxx:40:9: warning: declaration uses identifier '__OPTIONS_H__', which is a reserved identifier [bugprone-reserved-identifier]
   40 | #define __OPTIONS_H__
      |         ^~~~~~~~~~~~~
      |         OPTIONS_H_

Probably best dealt with across the whole project in one go

include/bout/options.hxx:263:13: warning: member variable 'value' has public visibility [cppcoreguidelines-non-private-member-variables-in-classes]
  263 |   ValueType value;
      |             ^
include/bout/options.hxx:280:40: warning: member variable 'attributes' has public visibility [cppcoreguidelines-non-private-member-variables-in-classes]
  280 |   std::map<std::string, AttributeType> attributes;
      |                                        ^

Fixing these would be pretty easy but would be API change

include/bout/options.hxx:335:20: warning: member 'match' of type 'const Options &' is a reference [cppcoreguidelines-avoid-const-or-ref-data-members]
  335 |     const Options& match;
      |                    ^

Not sure best fix for this, making it a pointer maybe, but then we'd get a different complaint

include/bout/options.hxx:364:3: warning: operator=() should return 'Options&' [cppcoreguidelines-c-copy-assignment-signature,misc-unconventional-assign-operator]
  364 |   T operator=(T inputvalue) {
      |   ^

Another API break, but I'm not even sure if anywhere is using the return value from assignment

[github-actions]

clang-tidy made some suggestions

[github-actions]

clang-tidy review says "All clean, LGTM! 👍"