Add is_loading and is_saving values to cereal::{Input,Output}Archive
Add is_loading and is_saving values to cereal::{Input,Output}Archive

This provides compatibility for asymmetric serialize routines when transitioning from boost serialization and associated archive types.

See: https://github.com/USCiLab/cereal/issues/360

Example:
```
    template <typename Archive>
    void serialize(Archive &ar, const std::uint32_t version)
    {
        std::int16_t tmp = foo;
        if(Archive::is_loading::value)
        {
            ar & tmp;
            foo = tmp; // For illustration only
        }
        else
        {
            tmp = foo; // For illustration only
            ar & tmp;
        }
        ar & foo;
    }
```

adasworks/cereal,USCiLab/cereal,adasworks/cereal,USCiLab/cereal,abutcher-gh/cereal,abutcher-gh/cereal