Add fe(s::Symbol) for programmatic formula construction

[junyuan-chen]

When constructing a formula programmatically, calling Term inside fe is verbose. It seems that nothing is relying on having an AbstractTerm to be called by fe. So, I suggest simplifying the syntax by directly passing the Symbol s to fe.

It is also easier to add multiple fixed effects from an array like this: reduce(+, fe.([:State, :Year])).

[matthieugomez]

Thanks!