Add Arigthmetic Mean Interval characteristic calculator

[ChainsManipulator]

Add calculator function for Arithmetic Mean Interval characteristic for given intervals array using formula:
$$\Delta_a=\displaystyle \frac{1}{n} * \sum_{j=1}^{m} \sum_{i=1}^{n_j} \Delta_{ij}$$
Where $\Delta_{ij}$ is an interval and $n$ is a number of intervals in the sequence.

Examples

X = [2, 4, 2, 2, 4]
x_intervals = intervals(X, 'Start', 'Normal')
result = arithmetic_mean(x_intervals)
print(result)
> 1.8

X = ['B','B','A','A','C','B','A','C','C','B']
x_intervals = intervals(X, 'Start', 'Lossy')
result = arithmetic_mean(x_intervals)
print(result)
> 2.4286

X = ['B','B','A','A','C','B','A','C','C','B']
x_intervals = intervals(X, 'Start', 'Normal')
result = arithmetic_mean(x_intervals)
print(result)
> 2.6

X = ['B','B','A','A','C','B','A','C','C','B']
x_intervals = intervals(X, 'End', 'Normal')
result = arithmetic_mean(x_intervals)
print(result)
> 2.4

X = ['B','B','A','A','C','B','A','C','C','B']
x_intervals = intervals(X, 'Start', 'Redundant')
result = arithmetic_mean(x_intervals)
print(result)
> 2.5385

X = ['B','B','A','A','C','B','A','C','C','B']
x_intervals = intervals(X, 'Start', 'Cycle')
result = arithmetic_mean(x_intervals)
print(result)
> 3

---

X = ['C','C','A','C','G','C','T','T','A','C']
x_intervals = intervals(X, 'Start', 'Lossy')
result = arithmetic_mean(x_intervals)
print(result)
> 2.666666666666667

X = ['C','C','A','C','G','C','T','T','A','C']
x_intervals = intervals(X, 'Start', 'Normal')
result = arithmetic_mean(x_intervals)
print(result)
> 3.2

X = ['C','C','A','C','G','C','T','T','A','C']
x_intervals = intervals(X, 'End', 'Normal')
result = arithmetic_mean(x_intervals)
print(result)
> 2.8

X = ['C','C','A','C','G','C','T','T','A','C']
x_intervals = intervals(X, 'End', 'Redundant')
result = arithmetic_mean(x_intervals)
print(result)
> 3.142857142857

X = ['C','C','A','C','G','C','T','T','A','C']
x_intervals = intervals(X, 'End', 'Cycle')
result = arithmetic_mean(x_intervals)
print(result)
> 4

---

X = ['C','C','C','C'] 
x_intervals = intervals(X, 'Start', 'Lossy')
result = arithmetic_mean(x_intervals)
print(result)
> 1

X =['C','C','C','C']  
x_intervals = intervals(X, 'Start', 'Normal')
result = arithmetic_mean(x_intervals)
print(result)
> 1

X = ['C','C','C','C'] 
x_intervals = intervals(X, 'End', 'Normal')
result = arithmetic_mean(x_intervals)
print(result)
> 1

X = ['C','C','C','C'] 
x_intervals = intervals(X, 'End', 'Redundant')
result = arithmetic_mean(x_intervals)
print(result)
> 1

X = ['C','C','C','C'] 
x_intervals = intervals(X, 'End', 'Cycle')
result = arithmetic_mean(x_intervals)
print(result)
> 1

No intervals

X = ['C','G'] 
x_intervals = intervals(X, 'End', 'Lossy')
result = arithmetic_mean(x_intervals)
print(result)
> 0

X = ['A','C','G','T'] 
x_intervals = intervals(X, 'End', 'Lossy')
result = arithmetic_mean(x_intervals)
print(result)
> 0

X = [2, 1] 
x_intervals = intervals(X, 'End', 'Lossy')
result = arithmetic_mean(x_intervals)
print(result)
> 0