dsolve doesn't know order-reducing substitutions #15881

[Teut2711]

This is my first attempt to contribute to open source.

References to other Issues or PRs

Brief description of what is fixed or changed

Issue: #15881

I have made a function which uses substitution recursively until equation is linear(the equation should have derivatives of order >=1). Looking forward to get it reviewed and corrections to be marked out
so that I can modify it as per requirement.

Other comments

Release Notes

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This is my first attempt to contribute to open source. 

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#### References to other Issues or PRs
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#### Brief description of what is fixed or changed

Issue: https://github.com/sympy/sympy/issues/15881

I have made a function which uses substitution recursively until equation is linear(the equation should have derivatives of order >=1). Looking forward to get it reviewed and corrections to be marked out 
so that I can modify it as per requirement. 


#### Other comments


#### Release Notes

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