least squares poly regression

[StephenGemin]

First of multiple submissions for Issue #840

Regression class

1. least squares regression of nth order
2. output polynomial coefficients
3. plot result

[StephenGemin]

I've made some changes to the files in the matrix folder. I would like to use these algorithms within the polynomial regression. There are however, many bugs that are stopping me at the moment ☺.

For the commits after the force-push:

Within the matrix folder:

1. Removed init.py
2. Added pytests for matrix operations as it was difficult to see if things were working correctly just running it through print statements on the main file. There were edge cases that the algorithms did not account for.

Made the following changes to matrix_operation.py

1. added matrix subtraction to matrix_operation.py
2. added matrix size checks for addition and subtraction as there were previously no checks
3. fixed typo in matrix multiplication loop that was in Pull Request Fixed error in matrix multiplication that didn't multiple non-square … #898 on TheAlgorithms/Python. Note I merged the changes from Fixed error in matrix multiplication that didn't multiple non-square … #898 into this PR.
4. PEP8 styling changes