Support arithmetic operators when adding signals on Simulator

Description

In the ngspice simulation window, it would be nice to be able to plot signal A - signal B or things like that, not just the plain signals. PSPICE does that allowing the user to define a string / formula like: I(A)*1e6 - 3*I(B)

Or via a GUI window with arithmetic operators like in PSPICE, see:

This has been already mentioned in #2333 (closed) point 4, but I thought it is better to keep separate track of different issues so that they can be assigned different priorities.

Steps to reproduce

1. Open Simulation tool from eeschema
2. Try to plot the difference between two signals

KiCad Version

Application: Eeschema

Version: 5.99.0-unknown-525934881~106~ubuntu18.04.1, release build

Libraries:
	wxWidgets 3.0.4
	libcurl/7.58.0 OpenSSL/1.1.1 zlib/1.2.11 libidn2/2.0.4 libpsl/0.19.1 (+libidn2/2.0.4) nghttp2/1.30.0 librtmp/2.3

Platform: Linux 4.15.0-128-generic x86_64, 64 bit, Little endian, wxGTK, , 

Build Info:
	Date: Dec 11 2020 10:17:44
	wxWidgets: 3.0.4 (wchar_t,wx containers,compatible with 2.8) GTK+ 3.22
	Boost: 1.65.1
	OCC: 7.3.0
	Curl: 7.58.0
	ngspice: 33
	Compiler: GCC 7.5.0 with C++ ABI 1011

Build settings:
	KICAD_SCRIPTING=ON
	KICAD_SCRIPTING_MODULES=ON
	KICAD_SCRIPTING_PYTHON3=ON
	KICAD_SCRIPTING_WXPYTHON=ON
	KICAD_SCRIPTING_WXPYTHON_PHOENIX=ON
	KICAD_SCRIPTING_ACTION_MENU=ON
	KICAD_USE_OCC=ON
	KICAD_SPICE=ON