Check that `SHPC_dir` exists

Check that `SHPC_dir` exists at initialization of instance of class
SHPC. Before this commit, if `SHPC_dir` did not exist, there was an
error trying to create `n_slices.txt`, which was less clear.

Common/util_eddies.py

@@ -4,6 +4,7 @@ from os import path
 import numpy as np
 import sys
 import bisect
+import os
 
 class SHPC_class:
     def __init__(self, SHPC_dir, def_orient):
@@ -13,6 +14,9 @@ class SHPC_class:
 
         """
 
+        if not os.access(SHPC_dir, os.R_OK):
+            sys.exit(f"{SHPC_dir} not readable")
+
         self.dir = SHPC_dir
         self.def_orient = def_orient
         fname = path.join(SHPC_dir, "n_slices.txt")