corrected switched assignment of IndicesL and IndicesR

the following would give obvious wrong results:
```
In [1]: from pytriqs.gf.local import GfImFreq

In [2]: A=GfImFreq(indicesL=['L'],indicesR=['R'],beta=50)

In [3]: A
Out[3]: GfImFreq g: indicesL = ['R'], indicesR = ['L']                                                                               
```
Even worse, if you give differnt number of indices for L and R, then N1 and N2 (determined correctly) and indicesL, indicesR would not be consistent. The assignment now is consistent with the return https://github.com/TRIQS/triqs/blob/master/pytriqs/gf/local/tools.py#L77