refactor linear_coefficients
proceed if c1 or c2 is not zero
Added helper functions, new tests
Made changed
Don't do costly expand() on top of Order.__new__() method (issue 3654)

    Also, change compute_leading_term -> as_leading_term, as
    compute_leading_term(x) for x*(1 + x**2)**10000 calls expand() too.
    This seems to be related to Add._eval_nseries method.  Sometimes,
    it doesn't return order term (e.g. (1 + x**2).nseries(x)) - that's
    why Pow._eval_nseries() does actually expand_multinomial() for
    (1 + x**2)**1000.
Add _eval_as_leading_term() for polygamma

    fix test failure O(1/digamma(1/x)) == O(1/log(x))
    in test_order.py
Added test for issue 3654
Cleanup around expr.as_leading_term calculation
minor edits to ode.py
Merge pull request #1930 from skirpichev/3654-no-expand-in-Order

issue 3654: Don't do costly expand() on top of Order.__new__() method
Merge pull request #5 from smichr/lincoff

minor edits to ode.py
renamed ``roots_quartic_euler`` to ``_roots_quartic_euler``
modified its arguments to avoid useless computations
Work around ODE test
standardize almost_linear code; reduce linear_coeff tests
small changes in the documentation.
Merge pull request #1947 from pernici/quartic

simpler solutions for a class of quartic equations
Merge pull request #1940 from Manoj-Kumar-S/lincoff

Differential equations with linear coefficients
Merge pull request #1950 from smichr/lincoff

standardize almost_linear code; reduce linear_coeff tests
Merge pull request #1901 from thisch/blacklist_doctest

issue 3650; doctest dependency check
fix a typo and lack of spaces in strings split across two lines