Classification for system of odes
Amendments on checking of linearity
Check for system of eq being solvable under classify_sysode

This PR contain methods to classify whether a given system of equation
is solvable by current solver. It is broadly divided on the basis of
parameters returned by classify_sysode which are linearity, no of eq
and order. If the return parameter by these methods is any "type" then
the given system is solvable otherwise None is returned which means
it is not solvable by current solving engine.
Added method to solve linear, 1st order and 2 eq system

This PR contain method to solve "System of two linear homogeneous first-order constant-coefficient ordinary differential equations". It solved by differentiating first equation and replacing the value of y' from second equation to the equation obtained. The second order equation is obtained on x(t) which is then solved and the value of x(t) is put on second equation thus obtaining a first order equation in y(t). This equation upon solving give value of y(t).:
Modified func_coef
Removed whitespace
Merge pull request #7489 from KDMOE/classify_sysode_linear_system_2eq

Classification for system of odes
Add Faisal Anees as authors
Merge pull request #7568 from hargup/faisal_anees

Add Faisal Anees as authors
Issue #7537: Add is_disjoint() method and an alias isdisjoint()

Add a method for finding if two sets are disjoint.
Merge pull request #7565 from amitsaha/isdisjoint

Issue #7537: Add is_disjoint() method and an alias isdisjoint()
Added basic vector tests
Merge branch 'master' of https://github.com/sympy/sympy into basicvector