adjust bounds of power transformer fit (#446)

* adjust bounds of power transformer fit

* add comments

mesmer/mesmer_m/power_transformer.py

@@ -143,8 +143,9 @@ def _neg_log_likelihood(coeffs):
         ]
         local_yearly_T = local_yearly_T[~np.isnan(local_yearly_T)]
 
-        # choosing bracket -2, 2 like for boxcox
-        bounds = np.c_[[0, -0.1], [1, 0.1]]
+        # first coefficient only positive for logistic function
+        # second coefficient bounded to avoid very steep function
+        bounds = np.array([[0, np.inf], [-0.1, 0.1]])
         # first guess is that data is already normal distributed
         first_guess = np.array([1, 0])