fix(mtssm.py): handle 1st stress period with incrch equal to -1 (#780)

* fix(mtssm.py): handle 1st stress period when no crch data

Initializes crch with a scalar of 0.0 rather than a Transient2d obj equal to 0
Using a test dataset from #743 that was demonstrating the problem
Updated an autotest to load and write an SSM file with no data specified for crch

#743

* fix(mtssm.py): fix multi-species handling of incrch

autotest/t068_test_ssm.py

@@ -144,9 +144,45 @@ def test_none_spdtype():
         success, buff = mf.run_model(report=True)
         assert success
 
+def test_ssm_readwrite():
+
+    # Instantiate MODFLOW model
+    mf = flopy.modflow.Modflow()
+
+    # These dimensions work for the SSM file to be loaded
+    dis = flopy.modflow.ModflowDis(mf, nlay=10, nrow=118, ncol=153,
+                                   nper=100, delr=100, delc=100, perlen=1)
+    bas = flopy.modflow.ModflowBas(mf)
+
+    # Instantiate MT3D model
+    mt = flopy.mt3d.Mt3dms(modflowmodel=mf, modelname='tran_v1', 
+                           version='mt3d-usgs')
+    btn = flopy.mt3d.Mt3dBtn(mt)
+
+    # Point to ssm file for test load and write
+    pth = os.path.join('..','examples','data','ssm_load_test')
+    fl = os.path.join(pth, 'tran_v1_b1.ssm')
+
+    # Check that example input file with no data specified for crch works
+    # (file comes from: https://github.com/modflowpy/flopy/issues/743)
+    ssm = flopy.mt3d.Mt3dSsm.load(fl, mt)
+
+    # Change to dedicated work directory
+    cwd = os.getcwd()
+    if not os.path.exists(os.path.join('temp','t068_ssm_write')):
+        os.makedirs(os.path.join('temp','t068_ssm_write'))
+
+    os.chdir(os.path.join('temp','t068_ssm_write'))
+
+    # Ensure file is writeable
+    ssm.write_file()
+
+    # Return to starting directory
+    os.chdir(cwd)
 
 if __name__ == '__main__':
     test_mt3d_ssm_with_nodata_in_1st_sp()
     test_none_spdtype()
+    test_ssm_readwrite()