Gamma_method() is broken for Obs that are NaN

[s-kuberski]

Commit #152 breaks the gamma_method for Obs that are NaN.
Minimal example:

o = pe.pseudo_Obs(1, .1, 'test')
o.gamma_method()
no = np.nan * o
no.gamma_method()

works fine in the old version but now throws an error:

---------------------------------------------------------------------------
ValueError                                Traceback (most recent call last)
<ipython-input-2-bd59fd2172b6> in <module>
      2 o.gamma_method()
      3 no = np.nan * o
----> 4 no.gamma_method()

~/phd/git/pyerrors_github/pyerrors/obs.py in gamma_method(self, **kwargs)
    323                     for n in range(1, w_max):
    324                         if g_w[n - 1] < 0 or n >= w_max - 1:
--> 325                             _compute_drho(gapsize * n)
    326                             n *= gapsize
    327                             self.e_tauint[e_name] = self.e_n_tauint[e_name][n] * (1 + (2 * n / gapsize + 1) / e_N) / (1 + 1 / e_N)  # Bias correction hep-lat/0306017 eq. (49)

~/phd/git/pyerrors_github/pyerrors/obs.py in _compute_drho(i)
    290 
    291             def _compute_drho(i):
--> 292                 tmp = self.e_rho[e_name][i + 1:w_max] + np.concatenate([self.e_rho[e_name][i - 1::-1], self.e_rho[e_name][1:w_max - 2 * i]]) - 2 * self.e_rho[e_name][i] * self.e_rho[e_name][1:w_max - i]
    293                 self.e_drho[e_name][i] = np.sqrt(np.sum(tmp ** 2) / e_N)
    294 

ValueError: operands could not be broadcast together with shapes (0,) (500,) 

This had not been the case before. However, If I revert the change, I can create a similar behavior when I add

o.idl['test'] = [1, 5] + list(range(7, 2002, 2))
o.gamma_method()
no = np.NaN * o
no.gamma_method()

, i.e., when I use a gapped MC history with a hole, the following exception is thrown:

---------------------------------------------------------------------------
ValueError                                Traceback (most recent call last)
<ipython-input-3-4fdb987095af> in <module>
      8 o.gamma_method()
      9 no = np.NaN * o
---> 10 no.gamma_method()

~/phd/git/pyerrors_github/pyerrors/obs.py in gamma_method(self, **kwargs)
    323                     for n in range(1, w_max):
    324                         if n < w_max // 2 - 2:
--> 325                             _compute_drho(gapsize * n + gapsize)
    326                         if g_w[n - 1] < 0 or n >= w_max - 1:
    327                             n *= gapsize

~/phd/git/pyerrors_github/pyerrors/obs.py in _compute_drho(i)
    290 
    291             def _compute_drho(i):
--> 292                 tmp = self.e_rho[e_name][i + 1:w_max] + np.concatenate([self.e_rho[e_name][i - 1::-1], self.e_rho[e_name][1:w_max - 2 * i]]) - 2 * self.e_rho[e_name][i] * self.e_rho[e_name][1:w_max - i]
    293                 self.e_drho[e_name][i] = np.sqrt(np.sum(tmp ** 2) / e_N)
    294 

ValueError: operands could not be broadcast together with shapes (499,) (500,)