Set required status in Field constructor

Signed-off-by: Jussi Virtanen <jussi.virtanen@valotrading.com>
Set required status in RepeatingGroup constructor

Signed-off-by: Jussi Virtanen <jussi.virtanen@valotrading.com>
Remove 'Field#setRequired'

Signed-off-by: Jussi Virtanen <jussi.virtanen@valotrading.com>
Rename 'Field#isRequired' to 'required'

Signed-off-by: Jussi Virtanen <jussi.virtanen@valotrading.com>
Remove 'Field#hasSingleTag'

Signed-off-by: Jussi Virtanen <jussi.virtanen@valotrading.com>
Remove InvalidValueForTagException

Use InvalidValueException instead.

Signed-off-by: Jussi Virtanen <jussi.virtanen@valotrading.com>
Remove 'Field#isFormatValid'

Instead of setting a flag if 'Field#parse' fails and later throwing an
exception if the flag is set, throw an InvalidValueFormatException as
soon as parsing fails.

Signed-off-by: Jussi Virtanen <jussi.virtanen@valotrading.com>
Remove 'Field#isValueValid'

Signed-off-by: Jussi Virtanen <jussi.virtanen@valotrading.com>
Enhance InvalidValueFormatException message

Signed-off-by: Jussi Virtanen <jussi.virtanen@valotrading.com>
Enhance InvalidValueException message

Signed-off-by: Jussi Virtanen <jussi.virtanen@valotrading.com>
Remove 'Field#isMissing'

Signed-off-by: Jussi Virtanen <jussi.virtanen@valotrading.com>
Remove 'Field#isConditional'

Signed-off-by: Jussi Virtanen <jussi.virtanen@valotrading.com>
Remove 'Field#isEmpty'

Signed-off-by: Jussi Virtanen <jussi.virtanen@valotrading.com>
Remove 'Field#prettyName'

Signed-off-by: Jussi Virtanen <jussi.virtanen@valotrading.com>
Merge pull request #131 from jvirtanen/cleanups/field

Clean up Field