title: C# Datatable - group by multiple columns with linq
body: <p>I have a Datatable like this</p>
  
  <pre><code>| Supplier  | Product | Price1 | Price2 | Price3 | ... | PriceN |
  |-----------|---------|--------|--------|--------|-----|--------|
  | Supplier1 | Orange  | 100    | 105    | 150    | ... | 180    |
  | Supplier1 | Orange  | 110    | 130    | 140    | ... | 180    |
  | Supplier2 | Orange  | 200    | 250    | 270    | ... | 350    |
  | Supplier2 | Orange  | 250    | 270    | 320    | ... | 270    |
  </code></pre>
  
  <p>I want to group rows as next:</p>
  
  <pre><code>| Supplier  | Product | Price1  | Price2  | Price3  | ... | PriceN  |
  |-----------|---------|---------|---------|---------|-----|---------|
  | Supplier1 | Orange  | 100-110 | 105-130 | 140-150 | ... | 180     |
  | Supplier2 | Orange  | 200-250 | 250-270 | 270-320 | ... | 270-350 |
  </code></pre>
  
  <p>Count of columns like "PriceN" can be arbitrary.
  How can I do this with LINQ?</p>