fix: Make repeating groups readable and writable

Although Stirling has had support for parsing (and, in principle,
formatting) repeating groups for a long time, neither getting nor
setting field values within a repeating group has been possible.

Add 'FieldContainer#getGroup' that provides access to a repeating
group identified by the tag giving the number of repeating group
instances, such as NoLegs(555) in FIX 4.3.

Signed-off-by: Jussi Virtanen <jussi.virtanen@valotrading.com>
Merge pull request #93 from jvirtanen/features/repeating-group

Make repeating groups readable and writable