Adds the basic header with simple information

Fixes #10 (closed) (at least for the gridded searches)

This adds a basic header with the data, username, and hostname information. Example output (from examples/transient_examples/short_transient_search_gridded.py)

# date:2017-12-20 11:54:51.736768;user:greg;hostname:greg-len-ubuntu
# _ _ F0 F1 F2 Alpha Delta
1.000000000000000000e+09 1.000172800000000000e+09 2.999998991056105524e+01 -1.000000000000000036e-10 0.000000000000000000e+00 5.000000000000000000e-01 1.000000000000000000e+00 4.206904220581054688e+01 1.000097200000000000e+09 1.980000000000000000e+04
1.000000000000000000e+09 1.000172800000000000e+09 2.999999011234983470e+01 -1.000000000000000036e-10 0.000000000000000000e+00 5.000000000000000000e-01 1.000000000000000000e+00 4.204999923706054688e+01 1.000097200000000000e+09 1.980000000000000000e+04

@dkeitel Am I missing anything from the header?

pyfstat/grid_based_searches.py

@@ -5,6 +5,9 @@ import os
 import logging
 import itertools
 from collections import OrderedDict
+import datetime
+import getpass
+import socket
 
 import numpy as np
 import matplotlib
@@ -186,10 +189,21 @@ class GridSearch(BaseSearchClass):
         if return_data:
             return data
         else:
-            logging.info('Saving data to {}'.format(self.out_file))
-            np.savetxt(self.out_file, data, delimiter=' ')
+            self.save_array_to_disk(data)
             self.data = data
 
+    def get_header(self):
+        header = ';'.join(['date:{}'.format(str(datetime.datetime.now())),
+                           'user:{}'.format(getpass.getuser()),
+                           'hostname:{}'.format(socket.gethostname())])
+        header += '\n' + ' '.join(self.keys)
+        return header
+
+    def save_array_to_disk(self, data):
+        logging.info('Saving data to {}'.format(self.out_file))
+        header = self.get_header()
+        np.savetxt(self.out_file, data, delimiter=' ', header=header)
+
     def convert_F0_to_mismatch(self, F0, F0hat, Tseg):
         DeltaF0 = F0[1] - F0[0]
         m_spacing = (np.pi*Tseg*DeltaF0)**2 / 12.