Need configurable Experiment Types

[katyjg]

In core.lims.models.Group, experiment types are currently hard-coded:

EXP_TYPES = Choices(
    (0, 'NATIVE', _('Native')),
    (1, 'MAD', _('MAD')),
    (2, 'SAD', _('SAD')),
    (3, 'S_SAD', _('S-SAD'))
)

This will need to be abstracted into a separate model for other beamlines to properly configure.