Internal helper function to returns bp_limit unless
it is the default c(-Inf, Inf), in which case the NPDA (Fourth Report)
or NDA (NICE/BHF) range for valid$bp_type is used.
See also
Other BP functions:
.bp_message(),
.bp_validate_demo_input(),
bp_assess(),
bp_category(),
bp_expected(),
bp_family,
bp_relative()