Import fails for Philips dose info import when ExposureDoseSequence has UN VR

[Ed McDonagh]

Not clear yet if this is a one-off or a wider problem, but the sequence has VR of UN rather than SQ, and therefore pydicom can't encode/interrogate the contents correctly.

No fix, but the import can be changed to not fail so badly.

[Ed McDonagh]

Modified ct_philips to write to the log but not fall over if no events are found in the file. Refs #302

→ <<cset 4af8c2dc1555>>

[Ed McDonagh]

Improved the usefulness of the warning log message from Philips dose info image objects if the series information can't be read. Fixes #302 as far as it is going to be.

→ <<cset f4dee42e1915>>

[Ed McDonagh]

Added ref #302 to the changes docs

→ <<cset 332b6c86cdd6>>