ENH: Allow Get*FromTag to accept uppercase hex

Change GetLabelFromTag and GetValueFromTag to allow
for both lower and upper case hex.

Also update the documentation to use an example tag with
hexadecimal-range digits.

Change-Id: Id427a2b6af6f39aaa961135094d15a11fea6d40c
Merge topic 'gdcm-tag'

9935aba ENH: Allow Get*FromTag to accept uppercase hex