return avg function value as bigdecimal for improving accuracy

[a5-stable]

Greetings 🙇‍♀️
I am having trouble getting an accurate result when using the AVG function with decimal values。
For example, when I run AVG(2, 3),

• I expect the result to be 2.5,
• but the actual result is 2.

I have made modifications to the code to ensure that it always returns a BigDecimal value.

before

c = Dentaku::Calculator.new
c.evaluate!("AVG(2, 3)")
#=> 2

c.evaluate!("AVG(2, 3.1)")
#=> 0.255e1

after

c = Dentaku::Calculator.new
c.evaluate!("AVG(2, 3)")
#=> 0.25e1

c.evaluate!("AVG(2, 3.1)")
#=> 0.255e1

[rubysolo]

Good catch, thank you!