BUG: LPP vetter needs to play nice with refactored Tce class

[pllim]

Follow up of #26 and #33 .

This needs fixing:

exovetter/exovetter/lpp.py

Lines 351 to 356 in c8b64ef

	# FIXME: This looks more correct but fails the test.
	# from exovetter import const as exo_const
	# self.tzero = tce.get_epoch(
	# getattr(exo_const, lc.time_format)).to_value(u.day)
	self.tzero = tce['epoch'].to_value(u.day)

This test needs to be updated:

exovetter/tests/test_lpp.py

Line 18 in c8b64ef

tzero = (54953.6193 + 2400000.5 - 2454833.0) * u.day

But I don't know what is going on with tzero in the test, so maybe @mustaric or @fergalm can fix it properly.