Pylops

* Changed internal behaviour of ``pylops.sparsity.OMP`` when
`niter_inner=0`. Automatically reverts to Matching Pursuit algorithm.
* Changed handling of `dtype` in ``pylops.signalprocessing.FFT`` and
``pylops.signalprocessing.FFT2D`` to ensure that the type of the input
vector is retained when applying forward and adjoint.
* Added `dtype` parameter to the `FFT` calls in the definition of the
``pylops.waveeqprocessing.MDD`` operation. This ensure that the type
of the real part of `G` input is enforced to the output vectors of the
forward and adjoint operations.

* Added ``pylops.waveeqprocessing.Deghosting`` and
``pylops.signalprocessing.Seislet`` operators
* Added hard and half thresholds in ``pylops.optimization.sparsity.ISTA``
and ``pylops.optimization.sparsity.FISTA`` solvers
* Added ``prescaled`` input parameter to ``pylops.waveeqprocessing.MDC``
and ``pylops.waveeqprocessing.Marchenko``
* Added sinc interpolation to ``pylops.signalprocessing.Interp``
(``kind == 'sinc'``)
* Modified ``pylops.waveeqprocessing.marchenko.directwave`` to
to model analytical responses from both sources of volume injection
(``derivative=False``) and source of volume injection rate
(``derivative=True``)
* Added ``pylops.LinearOperator.asoperator`` method to
``pylops.LinearOperator``
* Added ``pylops.utils.signalprocessing.slope_estimate`` function
* Fix bug in ``pylops.signalprocessing.Radon2D`` and
``pylops.signalprocessing.Radon3D`` when ``onthefly=True`` returning the
same result as when ``onthefly=False``

* 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/PyLops/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