add support for calculated_default attribute

[nnutter]

From the POD,

calculated_default is a name of an instance method or a code ref to be used to resolve a default value during object creation. The instance method will be called immediately after UR creates the initial entity so the method will have access to other parameters used during creation.

Specifying calculated_default => 1 is equivalent to,

calculated_default => '__default_' . $prop_name . '__'

and is meant to establish a naming convention without requiring it.

[brummett]

This should be if (exists $params->{$name}) { to allow specifying a false-y value in create()

[brummett]

Needs some documentation in UR/Object/Type/Initializer.pod

[brummett]

default_from => 1 reads pretty poorly. I thought about lazy_default => 1. That reads better (and works for the other use cases), but "lazy" doesn't really describe what's going on.

[nnutter]

default_from => 1 reads pretty poorly. I thought about lazy_default => 1. That reads better (and works for the other use cases), but "lazy" doesn't really describe what's going on.

Brainstorming...

• dynamic_default => 1
• default_at_construction => 1
• default_at_create =>
• calculated_default => 1

[nnutter]

FYI, I implemented Genome::Configurable with this, genome/genome@9243b8b.

[nnutter]

I pushed commits to address feedback so far (besides picking a better name than default_from).

[nnutter]

Pushed commits to rename default_from to calculated_default.

[brummett]

+1 after tests pass