can not assume most of dicom fields are there

It is only safe to assume we have study, series, and sop uids...
we should make sure the other ones are available.

```
chris@chris:/neuro/users/chris/log $>cd /neuro/users/chris/data/MR_SERVICE_PATIENT_ID_1-1545/2015.03.20-055Y-20150320_0642-3021/097_HeadMatrix_H1P_H2P_H3P_H4P_Tra_sn-097_HeadMatrix_H1P_H2P_H3P_H4P_Tra_sn-35027; /net/tautona/neuro/labs/grantlab/users/chris/src/chrisreloaded/lib/med2image/med2image.py -i $(/bin/ls -1 *dcm | head -n 1) -o %inputFile%SeriesDescription%ProtocolName -t jpg -s m
Mon Mar 23 05:58:19 2015 chris med2image.py[11072]: Converting DICOM image.
Mon Mar 23 05:58:19 2015 chris med2image.py[11072]: PatientName: SERVICE PATIENT
Traceback (most recent call last):
File "/net/tautona/neuro/labs/grantlab/users/chris/src/chrisreloaded/lib/med2image/med2image.py", line 568, in
C_convert.run()
File "/net/tautona/neuro/labs/grantlab/users/chris/src/chrisreloaded/lib/med2image/med2image.py", line 250, in run
self._log('PatientAge: %s\n' % self._dcm.PatientAge)
File "/usr/lib/python2.7/dist-packages/dicom/dataset.py", line 248, in __getattr__
"'{0:s}'.".format(name))
AttributeError: Dataset does not have attribute 'PatientAge'.
```

Start in lib/med2image/med2image.py at line 250, where the conversion run accesses PatientAge, and inspect the other DICOM fields used there. Reproduce the reported command with a DICOM file missing optional fields. Done means conversion no longer fails when fields beyond study, series, and SOP UIDs are absent.

Written by the indexing model from the issue text.