t_0 of LogRGSpaces may be any real number

Since t_0 = log(k_1), where k_1 is the first mode in the corresponding harmonic
space. k_1 can be smaller than one and therefore t_0 can be any real number.

nifty5/domains/log_rg_space.py

@@ -52,8 +52,6 @@ class LogRGSpace(StructuredDomain):
         self._t_0 = tuple(t_0)
         if min(self._bindistances) <= 0:
             raise ValueError('Non-positive bindistances encountered')
-        if min(self._t_0) <= 0:
-            raise ValueError('Non-positive t_0 encountered')
 
         self._dim = int(reduce(lambda x, y: x*y, self._shape))
         self._dvol = float(reduce(lambda x, y: x*y, self._bindistances))