update age category instance definitions and add death stage

[lzehl]

adolescent:
Life cycle stage of a subject loosely defined by the transitional growth
and development between childhood and adulthood, often described as ‘puberty’.

adult:
Life cycle stage of a subject that starts with sexual maturity and ends with
death.

death:
Life cycle stage of a subject that starts with death and ends with full
decomposition of the subject's body.
UBERON - http://purl.obolibrary.org/obo/UBERON_0000071

embryo:
Life cycle stage of a subject that starts with fertilization and ends with the
fully formed embryo.

infant:
Life cycle stage of subjects (mammals) that follows the neonate stage and ends at
weaning.

juveline:
Life cycle stage of a subject that starts with the independence of the nest
and/or caregivers and ends with sexual maturity.

late_adult:
Life cycle stage of a subject that follows the prime adult stage and ends with
death.

neonate:
Life cycle stage of a subject that immediately follows birth.

perinate: (NAME CHANGE WHICH REQUIRES EBRAINS CLEANUP; old: perinatal)
Life cycle stage of a subject that starts right before and ends right after birth.

prime_adult:
Life cycle stage of a subject that starts with sexual maturity or
the cessation of growth, whichever comes last, and ends before senescence.

young_adult:
Life cycle stage of a subject that starts with sexual maturity and ends with the
cessation of growth (if this is reached after sexual maturity).