Automatically convert DateTime to julian time as needed

```julia
using Dates
using AstroLib
sunpos(now())
```
currently throws `ERROR: MethodError: no method matching sunpos(::DateTime)`. Defining `sunpos(dt::DateTime) = Dates.datetime2julian(dt)` would implement this. This could be defined in a package extension with Dates, but AstroLib already has a full dependency on Dates so there's no need for that.