test(enums): fix enums doctests

[bonjourmauko]

Depends on #1272
Fixes #1267

New features

• Introduce indexed_enums.EnumType
  • Allows for actually fancy indexing indexed_enums.Enum

Technical changes

• Fix doctests
• Fix bug in Enum.encode when passing a scalar
  • Still raises TypeError but with an explanation of why it fails
• Fix bug in Enum.encode when encoding values not present in the enum
  • When encoding values not present in an enum, Enum.encode always encoded the first item of it
  • Now, it correctly encodes only the values requested that exist in the enum

Before

from openfisca_core import indexed_enums as enum

class TestEnum(enum.Enum):
     ONE = "one"
     TWO = "two"

TestEnum.encode([2])
#  EnumArray([0])

After

from openfisca_core import indexed_enums as enum

class TestEnum(enum.Enum):
     ONE = "one"
     TWO = "two"

TestEnum.encode([2])
#  EnumArray([])

TestEnum.encode([0,1,2,5])
# EnumArray([<TestEnum.ONE: 'one'> <TestEnum.TWO: 'two'>])

[coveralls]

coverage: 82.236% (+0.4%) from 81.804%
when pulling b5b7968 on test/fix-enums-doctests
into df9e9d7 on master.