Using linear interpolation to transition between shape functions.

Linear interpolation function between a and b.

```
The transition is made by changing the value of t within the range:
{ 0 <= t <= 1 }
result = (1 - t) * a + t * b
where:
t=0 => result = a
t=1 => result = b
```

This can be used to transition between two shape functions, a and b:

```
a = f_1(d_1, d_2, ..., d_n)
b = f_2(d_1, d_2, ..., d_n)
result = (1 - t) * f_1(d_1, d_2, ..., d_n) + t * f_2(d_1, d_2, ..., d_n)
Where:
{ 0 <= t <= 1 }
d_1 = x
d_2 = y
d_n = highest_dimension
```

Finally, this can be generalised to allow blends between N shape functions:

```
T = t_1 + t_2 + t_3 + ... + t_N
T = 1 at any transition position.
Where N is the number of shape functions.
result = t_1 * f_1 + t_2 * f_2 + t_3 * f_3 + ... + t_N * f_N
(or)
result =
t_1 * f_1(d_1, d_2, ..., d_n) +
t_2 * f_2(d_1, d_2, ..., d_n) +
t_3 * f_3(d_1, d_2, ..., d_n) +
... + t_N * f_N(d_1, d_2, ..., d_n)
```

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