Added tests to Substitution functions
Added tests to test both functionalities of simple formulas/terms of each type and corner cases for every substitution functions. Until issues #74, #80, and #81 are closed 4 tests are commented out
Added tests to test both functionalities of simple formulas/terms of each type and corner cases for every substitution functions. Until issues #74, #80, and #81 are closed 4 tests are commented out