SCUBA2: remove EXP_TIME and WEIGHTS uhdr entries

Previously these entries were used to artifically assert that the
corresponding NDF components existed within the current file. Now that
exp_time and weights components are explicitly checked before using
these flags can be removed.
general: new primitive to check a list of files for NDF component

A wrapper around _COMPONENT_EXISTS_ that accepts a list of files to
check. Warnings and errors are reported for files that do not contain
the given component.
SCUAB2: use new primitive to check NDF component for all files

Check whether exp_time and weights NDF components exist in each file
before aligning or adding.
SCUAB2: check NDF components exist before using them

The exp_time and weights components are now checked for existence
before using.