Added KroneckerDelta class. Added Tests.
Added API for LeviCivita class and its function definitions

Stuff to be done:
* complete the levicivita function
* update is_canonical accordingly
Updated function of `levi_civita` function. Added tests.

* Also `is_canonical` has been updated accordingly
Updated function decl of `has_dup` to `const vec_basic &arg`
Merge branch 'master' into tensor

Conflicts:
	src/functions.cpp
	src/tests/basic/test_functions.cpp
Updated function signature for Tensor Module.

* updated vec_basic arg to const vec_basic& arg
* updated vec_basic&& arg to const vec_basic&& arg
Merge pull request #267 from sushant-hiray/tensor

Tensor Module. Merged #267