Csdmpy

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What's new!
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- Added following methods to the ``CSDM`` class:
- ``__eq__()`` for all class
- ``__add__()`` = Adds two csdm object.
- ``__iadd__()`` = Adds two csdm objects in-place.
- ``__sub__()`` = Subtrace two csdm objects.
- ``__isub__()`` = Subtrace two csdm objects in-place.
- ``__mul__()`` = Multiply the components of the csdm object by a scalar.
- ``__imul__()`` = Multiply the components of the csdm object by a scalar in-place.
- ``__truvdiv__()`` = Divide the components of the csdm object by a scalar.
- ``__itruediv__()`` = Divide the components of the csdm object by a scalar
in-place.
- ``split()`` = Split the dependent-variables into individual csdm objects.

- Support for Numpy dimension reduction functions
- ``sum()``: Sum along a given dimension.
- ``prod()``: Product along a given dimension.

- Support for Numpy ufunc functions:
- ``sin``, ``cos``, ``tan``, ``arcsin``, ``arccos``, ``arctan``, ``sinh``, ``cosh``,
``tanh``, ``arcsinh``, ``arccosh``, ``arctanh``, ``exp``, ``exp2``, ``log``,
``log2``, ``log10``, ``expm1``, ``log1p``, ``negative``, ``positive``, ``square``,
``absolute``, ``fabs``, ``rint``, ``sign``, ``conj``, ``conjugate``, ``sqrt``,
``cbrt``, ``reciprocal``

- Added apodization functions.
- ``sin``, ``cos``, ``tan``, ``arcsin``, ``arccos``, ``arctan``, ``exp``

Bug fixes
'''''''''

- Fixed a bug in ``cp.plot()`` method.

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- Added method to convert the frequency dimension to nmr dimensionless frequency ratio
with syntax, ``dimension.to('ppm', 'nmr_frequency_ratio')``, where `dimension` is a
LinearDimension object.
- The ``csdmpy.plot()`` method also displays the dimension index on the axis label.

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- Added ``to_dict()`` method to the CSDM, Dimension, and DependentVariable objects.

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- Fixed warning message when physical quantity name is not found in the astropy units
package.
- Added dumps and loads function to dump and load the data model as json serialized
string, respectively without serializing it to a file.

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- Add a required `unsigned_interger_type` for SparseSampling dimension.
- Fixed minor bugs.
- Added a tags attribute to the CSDmodel object.
- Changed 'sampling_interval' key to 'count'.
- Changed 'quantity' key to 'quantity_name'.
- Changed 'index_zero_value' key to 'coordinates_offset'.
- Changed 'fft_output_order' key to 'complex_fft'.
- Renamed IndependentVariable class to Dimension.
- Renamed LinearlySpacedDimension class to LinearDimension.
- Renamed ArbitrarilySpacedDimension class to MonotonicDimension.
- Added a reciprocal attribute to LinearDimension and MonotonicDimension classes.
- Removed the reverse attribute from all Dimension classes.
- Changed 'sampling_interval' keyword to 'increment'.
- Changed 'reference_offset' keyword to 'index_zero_value'.
- Changed 'linear_spacing' literal to 'linear'.
- Changed 'arbitrarily_sampled' literal to 'monotonic'.
- Changed the defining of the coordinates for the LinearDimension from

.. math::

X^\text{ref} = m_k J_k - c_k {\bf 1}

to

.. math::
X^\text{ref} = m_k J_k + c_k {\bf 1},

where :math:`c_k` is the reference offset, :math:`m_k` is the increment, and
:math:`J_k` is the set of integer indices along the dimension.

- Added 'description' key to 'Dimension', 'DependentVariable' and 'CSDM' object.
- Changed 'CSDM' keyword to 'csdm'
- Changed 'FFT_output_order' keyword to 'fft_output_order'
- Changed 'components_URL' keyword to 'components_url'