Default to the latest version noise template

[dwhan89]

Currently, users have to specify which noise template to use when initializing SimGen.

class SimGen(object):
    def __init__(self, version, model="act_mr3", cmb_type='LensedCMB', dobeam=True, add_foregrounds=True, apply_window=True, max_cached=1,
                 extract_region = None,
                 extract_region_shape = None,
                 extract_region_wcs = None):

But we never provide users with the list of available noise template versions. Either we provide the list for users to choose from, or we need to default to the latest version. I think the latter is a better option.