Handle SUM() formula expression

[kdinev]

Implement handling for a SUM() formula expression.

Requirements:

• SUM() should evaluate to a number
• SUM() should take as parameters a sequence of values, or value references
  • SUM(1, 2, 3) should yield 6
  • SUM(A1, A2, A3) should yield the sum of the reference values in elements marked with the corresponding IDs, or data-ref attributes
• SUM() should take as parameters range of values, or value references
  • SUM(1 : 3) should yield 6
  • SUM(A1:A3) should yield the sum of the reference values in elements marked with the corresponding IDs, or data-ref attributes