* Added ``eigs`` and ``cond`` methods to ``LinearOperator``
to estimate eigenvalues and conditioning number using scipy wrapping of
[ARPACK](http://www.caam.rice.edu/software/ARPACK/)
* Modified default ``dtype`` for all operators to be ``float64`` (or ``complex128``)
to be consistent with default dtypes used by numpy (and scipy) for real and
complex floating point numbers.
* Added ``Flip`` operator
* Added ``Symmetrize`` operator
* Added ``Block`` operator
* Added ``Regression`` operator performing polynomial regression
and modified ``LinearRegression`` to be a simple wrapper of
the former when ``order=1``
* Modified ``pylops.basicoperators.MatrixMult`` operator to work with both
numpy ndarrays and scipy sparse matrices
* Added ``pylops.avo.prestack.PrestackInversion`` routine
* Added data weight optional input to ``NormalEquationsInversion``
and ``RegularizedInversion`` solvers
* Added ``IRLS`` solver