Clean up and sync dependencies for pip/conda, optional requirement of numpy/scipy.

[azukov]

Currently conda installs extra packages numpy/scipy/matplotlib. They are not needed for PyORBIT core.
On the other hand #72 and #83 depend on numpy/scipy.
We probably want to cleanly run without numpy - requires some sort of try.. except around import xxxx_requiring_numpy
We also want to an optional requirement that installs numpy/scipy.

Also requirements.txt should be used only for development installation --no-build-isolation --editable

[austin-hoover]

I would vote to add NumPy as a required python package if it's used in the orbit module. What's the harm, as long as a specific version of NumPy is attached to a specific version of PyORBIT?

[austin-hoover]

Regarding #83: The core tracking routines do not use NumPy or SciPy. NumPy is used for linear algebra; some, but not all, operations could be rewritten using native PyORBIT Matrix class. SciPy is used for its optimization package to compute periodic (matched) envelope parameters; this is an extra feature that could be removed, although it is useful to have. Matplotlib is only used in the example scripts.