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
Fix `Basic.rewrite()` behavior

Fixes the behavior of `Basic.rewrite` to call the opropriate rule
function.  Furthermore, added handling for `deep` keyword to
`Basic.rewrite()`, which defaults to `True`.  This change allows the
removal of `Function.rewrite()` and `physics.quantum.QExpr.rewrite()`
methods, however, the default behavior is changed from `deep=False` to
`deep=True`.
Removed whitespace
Revert physics.quantum.QExpr.rewrite to fix spin doctest
Issue #7537: Add alias for some of the methods in sympy.sets

This commit adds aliases for some of the existing methods so as to be
compatible with Python's builtin set/frozenset type.
Merge pull request #7556 from flacjacket/rewrite_fix

Fix `Basic.rewrite()` behavior
Added egyptian_fraction function for Egyptian Fractions calculation
Merge pull request #7557 from skirpichev/4122_egyptian_fraction

Added egyptian_fraction function for calcualting Egyptian Fractions
Merge pull request #7553 from amitsaha/sets_compat

Issue #7537: CPython set/frozenset compatible FiniteSet interface
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
Implement basic Mathematica code printer