Clean up of directories, variable names, file organization, etc

[mattsignorelli]

I'm going through the directories now and feel there is a lot of convention clean up needed:

• CamelCase for file names should be reserved for Modules. For non-modules, just use snake_case
• Choose either snake-case or camel case, but not both, e.g. Subatomic_Particles dictionary
• All exports should be included near the top of AtomicAndPhysicalConstants.jl, not spread throughout the files
• Clarification of global constants, e.g. is Subatomic_Particles a global constant? If so, add const and call it SUBATOMIC_SPECIES for example (all caps snake-case for constants is my personal preference)
• Files and variable locations should be reorganized into some cleaner layout. For example, one way is to put constructors for each struct right underneath the struct definition. An alternative is to have a file called ctors.jl for example containing the constructors.
• Change AbstractString -> String? I don't see much benefit in allowing weird other kinds of strings. This is to improve type-stability of dictionary accessing

[rot4te]

I'm checking these off while I work on them, not necessarily pushed yet