Naming pattern for IAM functions

[cwhanse]

Is your feature request related to a problem? Please describe.
It may be easier for users to find and select IAM functions if the function names start with the prefix iam, e.g., iam_ashrae rather than the current pattern placing iam as a suffix ashraeiam

Describe the solution you'd like
Rename physicaliam, ashraeiam, etc. and deprecate old names.

[wholmgren]

Related to #436... I currently prefer to see a new module iam.py with functions physical, ashrae, etc.

[cwhanse]

Starting on a PR for this. iam.py conflicts with the often-used variable iam, e.g., iam = pvsystem.ashraeiam() in the current code. Suggestions? Seems like a good idea to keep iam available for users to name the output of these functions.

[wholmgren]

I don't have a better idea, but I don't think we need to worry about what variables might exist in user code. There's only a name conflict when the module is imported as from pvlib import iam. While that's a common pattern, I don't think it's frequent enough to justify choosing a worse name for our module.