Adding to plotting.smith() a MATLAB-like smith chart option
Use 'renumber' method to implement the 'flip' method.

This fixes a bug:  the flip method did not flip the port impedances.

I think we can now remove the network module function 'flip'.
Added examples to doc string for method Network.renumber()
Fixed bug in network.connect() method port impedance managment
added a Frequency.npoints setter, which was accidently removed.
minor change to save_all_figs
fixing Frequency, and Network to accept arbitrary frequency vectors

modified `Frequency` class's `from_f()` function, and changed
`Network.read_touchstone()` to use this.

created  a test_frequency test case to test the from_f and the
linear sweep init.

it would be nice to change the initializer to accept both a
start/stop/n_points or an vector, with  unit. instead of doing
the class method, but will have to think about it
minor change
last commit forgot to change the Frequency.copy() to allow for
arbitrary f's
added option `Network.write_touchstone()` to toggle the  writing of
z0 in comments
fixed bug with Network.read_touchstone() when reading a complex z0
as written by skrf
added calibrationSet.py as a default import in calibration module
added a forgotten test file
added touchstone test coverage

wrote test and a simple touchstone file to verify
the s-parameters are read in the correct way
fixed minor __mul__ bug in Network
fixed minor bug in media.

the shunt capacitor and shunt_inductor did not terminate the
shunted component to ground. now they do .
adding pickling ability to Network

added `__setstate__` to `Network` so that pickling is possible.
also added test coverage for this.
adding pickling support

added support and test coverage for Network and Calibration `pickle`
methods. also added generic `read` and `write` methods to `skrf.convenience`
which can pickle and unpickle files. tested this with
`Network`, `NetworkSet`, `Frequency`, and `Calibration` objects
big IO changes with new pickling support

added a io submodule. touchstone will be moved here and any other
IO functions. currently there is a `read` and `write` methods,
which uses the pickle module to read and write any skrf objects
with automatic extensions.  the extensions are only for the users
needs, meaning the read function doesnt using extensions at all.
added pickling support to Media and RectangularWaveguide