Advanced Subexpression Insertion

Moved a few methods for elimination of selected subexpressions from lbmpy to pystencils. Helpful to control the granularity of common subexpression elimination, expression tree cleanup, and potentially to simplify equations by substituting constant or zero subexpressions.

pystencils/simp/__init__.py

@@ -5,10 +5,17 @@ from .simplifications import (
     add_subexpressions_for_sums, apply_on_all_subexpressions, apply_to_all_assignments,
     subexpression_substitution_in_existing_subexpressions,
     subexpression_substitution_in_main_assignments, sympy_cse, sympy_cse_on_assignment_list)
+from .subexpression_insertion import (
+    insert_aliases, insert_zeros, insert_constants,
+    insert_constant_additions, insert_constant_multiples,
+    insert_squares, insert_symbol_times_minus_one)
 from .simplificationstrategy import SimplificationStrategy
 
 __all__ = ['AssignmentCollection', 'SimplificationStrategy',
            'sympy_cse', 'sympy_cse_on_assignment_list', 'apply_to_all_assignments',
            'apply_on_all_subexpressions', 'subexpression_substitution_in_existing_subexpressions',
            'subexpression_substitution_in_main_assignments', 'add_subexpressions_for_constants',
-           'add_subexpressions_for_divisions', 'add_subexpressions_for_sums', 'add_subexpressions_for_field_reads']
+           'add_subexpressions_for_divisions', 'add_subexpressions_for_sums', 'add_subexpressions_for_field_reads',
+           'insert_aliases', 'insert_zeros', 'insert_constants',
+           'insert_constant_additions', 'insert_constant_multiples',
+           'insert_squares', 'insert_symbol_times_minus_one']