Implement `homogeneous_lde`

Solves a system of homogeneous linear diophantine equations when given
in it's matrix form. This method returns the basis of the solution set
if the system is solvable. A solution is a vector of natural numbers.
Add tests
Overload `==` for MatrixBase types inside CSymPy namespace

Otherwise we can't get `std::find` to work for DenseMatrix types.
Add reference
Merge branch 'master' into homogeneous_lde

Conflicts:
	src/matrix.h
Refactor `homogeneous_lde` using zeros
Remove redundant `else` statement
Use `std::vector` instead of raw C arrays
Remove unsafe code as suggested by Ondrej

Original comments:
https://github.com/sympy/csympy/pull/335#discussion_r20880442
https://github.com/sympy/csympy/pull/335#discussion_r20880146
Check whether entries of `A` and `product` are integers
Add assert statements to verify that we are dealing with Integers

Assert statements are not going to slow anything in release mode and
will prevent segfaults in debug mode. Change was done following advice
from Ondrej. Original comment:
https://github.com/sympy/csympy/pull/335#issuecomment-64804701
Merge pull request #335 from thilinarmtb/homogeneous_lde

Homogeneous Linear Diophantine Equations
Refactor matrix tests to increase readability
Fix a typo
Replace iterators by `for` loops

All the other functions use `for` loops instead of iterators. It's better
to be consistent.
Remove comment

This comment is not valid anymore because we have a lot of original
implementations of number theory functions which are not implemented
in `gmp`.
Fix doxygen description
Merge pull request #338 from thilinarmtb/ntheory_refactor

Refactor `ntheory.h`
Merge pull request #337 from thilinarmtb/refactor

Refactor Matrix module
Implement `vec_dense_matrix_eq_perm`

This function checks whether two vectors containing dense matrices are
equal upto a permutation. This is essentially the same function as
`vec_basic_eq_perm` in src/dict.cpp with minor modifications to be used with
dense matrices and was implemented following a suggestion by Isuru.