Mix input types, e.g. scalars with arrays

Following discussion on fortran-lang (https://fortran-lang.discourse.group/t/feq-parse-updates-evaluation-with-arrays-now-supported/6871/9)

> Nice to see the developments for this package. I have not had an opportunity yet to use it in earnest, but I do have an application in mind. One thing, though, not criticism, just a remark: if you use it to evaluate an array, then all variables should be arrays, right? No possibilty to combine a scalar with an array? Something along these lines:
```
real :: a(100)
real :: b

...
expression = 'exp( -b * a)'
...
write(*,*) f%evaluate( x )
```