[Beer] Replace two theta mask function with easier to inspect and understand type

[jokasimr]

Currently in the beer workflow TwoThetaMask is a function angle -> bool.
This is very "flexible" but in practice not so useful.

Problems:

• Can not be inspected easily wf.compute(TwoThetaMask) returns function.main.lambda at xyzabc123.
• Harder to define for users.

Solution:

• Replace it with a tuple that defines the upper and lower limits.