Pylops

* Added ``todense`` method to ``pylops.LinearOperator``
* Added ``pylops.signalprocessing.Bilinear``,
``pylops.signalprocessing.DWT``, and
``pylops.signalprocessing.DWT2`` operators
* Added ``pylops.waveeqprocessing.PressureToVelocity``,
``pylops.waveeqprocessing.UpDownComposition3Doperator``, and
``pylops.waveeqprocessing.PhaseShift`` operators
* Fix bug in ``pylops.basicoperators.Kronecker``
(see [Issue 125](https://github.com/Statoil/pylops/issues/125))

* Added ``pylops.basicoperators.Gradient``,
``pylops.basicoperators.Sum``,
``pylops.basicoperators.FirstDirectionalDerivative``, and
``pylops.basicoperators.SecondDirectionalDerivative`` operators
* Added ``pylops._ColumnLinearOperator`` private operator
* Added possibility to directly mix Linear operators and numpy/scipy
2d arrays in ``pylops.basicoperators.VStack`` and
``pylops.basicoperators.HStack`` and
``pylops.basicoperators.BlockDiagonal`` operators
* Added ``pylops.optimization.sparsity.OMP`` solver

* Added ``pylops.signalprocessing.ConvolveND`` operator
* Added ``pylops.utils.signalprocessing.nonstationary_convmtx`` to create
matrix for non-stationary convolution
* Added possibility to perform seismic modelling (and inversion) with
non-stationary wavelet in ``pylops.avo.poststack.PoststackLinearModelling``
* Create private methods for ``pylops.basicoperators.Block``,
``pylops.avo.poststack.PoststackLinearModelling``,
``pylops.waveeqprocessing.MDC`` to allow calling different operators
(e.g., from pylops-distributed or pylops-gpu) within the method

* Added ``conj`` method to ``pylops.LinearOperator``
* Added ``pylops.basicoperators.Kronecker``,
``pylops.basicoperators.Roll``, and
``pylops.basicoperators.Transpose`` operators
* Added ``pylops.signalprocessing.Fredholm1`` operator
* Added ``pylops.optimization.sparsity.SPGL1`` and
``pylops.optimization.sparsity.SplitBregman`` solvers
* Sped up ``pylops.signalprocessing.Convolve1D`` using
``scipy.signal.fftconvolve`` for multi-dimensional signals
* Changes in implementation of ``pylops.waveeqprocessing.MDC`` and
``pylops.waveeqprocessing.Marchenko`` to take advantage of primitives
operators
* Added ``epsRL1`` option to ``pylops.avo.poststack.PoststackInversion``
and ``pylops.avo.prestack.PrestackInversion`` to include
TV-regularization terms by means of
``pylops.optimization.sparsity.SplitBregman`` solver

* Added ``numba`` engine to ``pylops.basicoperators.Spread`` and
``pylops.basicoperators.Radon2D`` operators
* Added ``pylops.signalprocessing.Radon3D`` operator
* Added ``pylops.signalprocessing.Sliding2D`` and
``pylops.signalprocessing.Sliding3D`` operators
* Added ``pylops.signalprocessing.FFTND`` operator
* Added ``pylops.signalprocessing.Radon3D`` operator
* Added ``niter`` option to ``pylops.LinearOperator.eigs` method
* Added ``show`` option to ``pylops.optimization.sparsity.ISTA`` and
``pylops.optimization.sparsity.FISTA`` solvers
* Added ``pylops.waveeqprocessing.seismicinterpolation``,
``pylops.waveequprocessing.waveeqdecomposition` and
``pylops.waveequprocessing.lsm`` submodules
* Added tests for ``engine`` in various operators
* Added documentation regarding usage of ``pylops`` Docker container

* Added ``fftw`` engine to ``FFT`` operator
* Added ``ISTA`` and ``FISTA`` sparse solvers
* Added possibility to broadcast (handle multi-dimensional arrays)
to ``Diagonal`` and ``Restriction`` operators
* Added ``Interp`` operator
* Added ``Spread`` operator
* Added ``Radon2D`` operator