ephemerides handling fixes

@GregAshton 2 fixes to ephemerides handling:

1. Fix a problem I introduced in !18 (merged) - for some reason I cannot remember (maybe just "high numbers look good") I picked DE421 as the standard to look for, while in lalapps it's currently actually DE405 for both CFSv2 and MFDv5.

2. Teach Writer.run_makefakedata to actually pass through the chosen ephem files to the MFDv5 command line. The search class already properly handles them through InitBarycenter, while for making SFTs it seems we never actually did this properly so far.

I'll also need to add instructions for manual ephemerides download to the README, until a better way to provide those to pip users can be figured out; but I'll do that after !23 (merged) to avoid conflicts.

(fixed-up version of !24 (closed))

cc @ReinhardPrix in case you have opinions on the default version?

pyfstat/core.py

@@ -159,6 +159,8 @@ def predict_fstat(
     IFOs=None,
     assumeSqrtSX=None,
     tempory_filename="fs.tmp",
+    earth_ephem=None,
+    sun_ephem=None,
     **kwargs
 ):
     """ Wrapper to lalapps_PredictFstat
@@ -201,6 +203,11 @@ def predict_fstat(
     cl_pfs.append("--maxStartTime={}".format(int(maxStartTime)))
     cl_pfs.append("--outputFstat={}".format(tempory_filename))
 
+    if earth_ephem is not None:
+        cl_pfs.append("--ephemEarth='{}'".format(earth_ephem))
+    if sun_ephem is not None:
+        cl_pfs.append("--ephemSun='{}'".format(sun_ephem))
+
     cl_pfs = " ".join(cl_pfs)
     helper_functions.run_commandline(cl_pfs)
     d = read_par(filename=tempory_filename)
@@ -345,7 +352,8 @@ class BaseSearchClass(object):
             Paths of the two files containing positions of Earth and Sun,
             respectively at evenly spaced times, as passed to CreateFstatInput
 
-        Note: If not manually set, default values in ~/.pyfstat are used
+        Note: If not manually set, default values from get_ephemeris_files()
+              are used (looking in ~/.pyfstat or $LALPULSAR_DATADIR)
 
         """
 
@@ -353,8 +361,12 @@ class BaseSearchClass(object):
 
         if earth_ephem is None:
             self.earth_ephem = earth_ephem_default
+        else:
+            self.earth_ephem = earth_ephem
         if sun_ephem is None:
             self.sun_ephem = sun_ephem_default
+        else:
+            self.sun_ephem = sun_ephem
 
 
 class ComputeFstat(BaseSearchClass):