This commit removed code obsoleted by c12b82e (MNT: Updated setup.py)

The packages required to successfully develop and
create binary distributions are added to dev-requirements.txt

This commit introduces a new dependencies to "scikit-built" and CMake.

scikit-build is a drop-in replacement to setuptools.setup function
allowing to easily compile and package extensions (C/C++/Cython) by
bridging CMake and setuptools. See http://scikit-build.org

CMake is is an open-source, cross-platform family of tools designed
to build, test and package software. See https://cmake.org


Currently, scikit-build and CMake have to be explicitly installed
on the system. Since CMake wheels (allowing to pip install CMake)
will be published shortly, initial will be simplified.


How does it work ?

In addition to simplifying setup.py, three new files have been added:

<root>
  |
  |---- CMakeLists.txt                          (1)
  .
  .
  .
  |--- src
  |     |--- CMakeLists.txt                     (2)
  .
  .
  |--- skbeam
  |     |---core
  .     |     |---accumulators
  .     .     .       |--------- CMakeLists.txt (3)



The first CMakeLists.txt specifies requirements for

* Python interpreter,

* the associated python libraries

* and a "PythonExtension" modules containing convenience CMake
  functions allowing to easily build extension by encapsulating
  system introspection and platform specific logic.

and include the subdirectory associated with the two other CMakeLists.txt.



The second CMakeLists.txt is specific to "ctrans" extension and
it specifies:

* requirement for NumPy

* optional dependency on OpenMP. If it is not found on a given platform,
  nothing is changed, the macro _OPENMP will NOT be defined and a regular
  implementation will be compiled in.

Then, it declares:

* a library, associate pytho extension specific properties

* and finally add an install rule specifying where in the package
  hierarchy the extension should be installed.



Finally, the third CMakeLists.txt is specific to the "histogram" extension
and it specifies:

* requirements for NumPy

Then, it adds "cythonizing" target for the pyx file using the CMake
function "add_cython_target", and proceed to the declaration
of the extension like it was done in the previous case.


Et voila,