IMSC reader: contentProfiles does not accept the `all(...)` syntax

[palemieux]

See https://www.w3.org/TR/ttml2/#profile-attribute-contentProfiles

ttconv/src/main/python/ttconv/imsc/attributes.py

Line 118 in a426e4e

_CONTENT_PROFILES_RE = re.compile(r"\S+(?:\s+\S+)*")