[DataFrame] DataFrame operations "obj" return type

Given a DataFrameColumn with data similar to the following:


Performing operations like `Sum` return values of type `obj`. This causes issues when trying to use it in other contexts / operations.

Example:

Suppose I want to manually calculate the sum of a numeric column and divide it by 3:

```fsharp
df.["PetalLength"].Sum() / 3
```

The code throws the following exception - **The type 'int' does not match the type 'obj'**

The first thought is to cast.

```fsharp
((float32)df.["PetalLength"].Sum())
```

This approache, throws exception - **typecheck error The type 'obj' does not support a conversion to the type 'float32**.

The two ways that actually work are the following:

Using the casting operator:

```fsharp
(df.["PetalLength"].Sum() :?> float32) / 3.f
```

or unboxing:

```fsharp
df.["PetalLength"].Sum() |> unbox
```

In a sense it's not really a bug, but the ability to provide the return type and avoid casting would make it cleaner / simpler on the user.

i.e. `df.[PetalLength].Sum()`