Add function for interpolated IAM model

[cwhanse]

Describe the solution you'd like

A commonly used model for IAM is to interpolate along a user-supplied list of (AOI, IAM) points, e.g., pvsyst offers this model option.

Implement a function iam_interpolated with parameters being a list of (AOI, IAM) points.

Additional context
pvsyst uses linear interpolation.

[wholmgren]

I'm not entirely opposed to adding a one-line wrapper around numpy or scipy interpolate 1D functions. An alternative to consider is only a documentation example that demonstrates using those functions for this purpose. Or ask and answer the question on stackoverflow.

[cwhanse]

Good point. I think it would be useful as a PVSystem method, when the (AOI, IAM) points are packaged with module_parameters.

[mikofski]

Or perhaps in the model chain? Is there a pvsyst model chain for user supplied IAM lookup?

[cwhanse]

closed by #752