Clongdouble alias (#713)

* add alias to support non-linux

Manually import clongdouble as complex256 because the alias exists only on linux x86_64

* ignore automatically generated file

The poisson_camma_mixtures.c file is always generated automatically. Let's ignore it in git.

.gitignore

@@ -36,3 +36,4 @@ pisa.egg-info
 
 # IPython notebook backups
 .ipynb_checkpoints
+pisa/utils/llh_defs/poisson_gamma_mixtures.c

pisa/__init__.py

@@ -18,8 +18,11 @@
     float32, float64,
     int0, int8, int16, int32, int64,
     uint0, uint8, uint16, uint32, uint64,
-    complex64, complex128, complex256,
+    complex64, complex128
 )
+# The alias only exists on Linux x86_64, see
+# https://numpy.org/devdocs/reference/arrays.scalars.html#numpy.clongdouble
+from numpy import clongdouble as complex256
 import numpy as np
 from pint import UnitRegistry