Linop

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* add BlockVert. and BlockHoriz. operators
* add aslinearoperator convenience function
* fix handling of sysmmetric flag for MatrixOperator
* fix support for custom loggin in BlockOperator
* fix setup script, now cleaner and uses setuptools
* more test coverage
* various bug fixes
* various pep8 fixes

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* add MatrixOperator
* add documentation
* bug fixes
* pep8 fixes

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* add test suite for main operators
* various bug fixes caught during unit testing
* API is now compatible with scipy's interface
* real and complex operators have .H, additional .T for real dtype
* ensure consistency between operator and operand

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* initial import of sources, forked from pykrylov.linop