some renames to draw the eWoms directory structure nearer to the the one of the remaining OPM modules

this renames the 'test' directory to 'tests' and 'test/implicit' to
'tests/models'. the latter change reflects the fact that in eWoms all
models are implicit since the IMPET models have been removed.

tests/models/grids/obstacle_24x16.dgf

@@ -0,0 +1,15 @@
+DGF
+Interval
+0 0   % first corner 
+60 40   % second corner
+24 16   % 24 cells in x and 16 in y direction
+#
+GridParameter
+overlap 0 
+periodic 
+closure green
+#GridParameter 
+BOUNDARYDOMAIN
+default 1    % all boundaries have id 1
+#BOUNDARYDOMAIN
+# unitcube.dgf