Support compound expressions for integrand

[KirillZubov]

@parameters t
@variables i(..)
Di = Differential(t)
Ii = Integral(t in DomainSets.ClosedInterval(0, t))

it is only processing a simple expression with the variable of NN

Ii(i(t))

Need support compound expressions.

Ii(i(t)+1)
Ii(i(t)*exp(t))

[killah-t-cell]

I'll try to tackle this. Probably involves a change to _transform_expression and build_symbolic_loss_function. Will also think about how we can support compound expressions with two variables

@parameters t
@variables i(..) j(..)
Di = Differential(t)
Ii = Integral(t in DomainSets.ClosedInterval(0, t))

Ii(i(t)*j(t))

It might need some more restructuring for that.

[xtalax]

I think pde_system_transformation! in MOL handles this, I'll move it to base.