This commit is intented to produce more human-readable schematics

Before this commit the attenuator schematics were too compact and it was difficult (or even impossible) to read the resistor values at first sight.

Also, the resistor values were rounded to 1 decimal place as it is more than enough for actual attenuator design.

It was added a checkbox in the qucsatenuattor UI in order to let the user to attach a SP simulation component to the attenuator circuit.

Resistive attenuators are wideband circuits so the GUI doesn't ask for a frequency sweep. 

The tool sets the SP sweep to 50MHz-3GHz. If needed, the user can change that before running the simulation.

It was added the reflection type synthesis to qucsattenuator.

The synthesis equations can be found here:
https://www.microwaves101.com/encyclopedias/reflection-attenuators

Added a new feature to the GUI to show the power dissipated by each resistor given the overall attenuation and the input power.

The QLineEdits used for the design input were replaced by QSpinBoxes. It seems to be more convenient and fluid since now the user only need to move the mouse wheel over the input to see how the resistor values move

The synthesis fails for certain Z0/att ratios in Tee and Pi type attenuators. However, success is guaranteed for the rest of attenuator types. In this sense, it makes sense to warn (in red) whenever the synthesis fails and remove the "Success" message.

This commits adds the capability to design quarter-wave series and shunt attenuators

A feature for synthesizing transmission lines with lumped components (lowpass form) is also added.

Reference:

The PIN diode circuit designer's handbook. W.E. Doherty, Jr., R.D. Joos, Microsemi Corp., 1998
Add QW series and QW shunt attenuators

This commits adds the capability to design quarter-wave series and shunt attenuators

A feature for synthesizing transmission lines with lumped components (lowpass form) is also added.

Reference:

The PIN diode circuit designer's handbook. W.E. Doherty, Jr., R.D. Joos, Microsemi Corp., 1998

The output of the L-pad attenuator is unmatched. The output impedance is shown in a QLineEdit.
The output impedance is shown in a box. The derivation of the design equations will be provided separately in the pull request

The output of the L-pad attenuators is unmatched. The output impedance is shown in a box.
The derivation of the design equations will be provided separately in the pull request

The series resistor attenuator is unmatched at both ports. The input and output impedance is shown in a box.
The derivation of the design equations will be provided separately in the pull request

The shunt resistor attenuator is unmatched at both ports. The input and output impedance is shown in a box.
The derivation of the design equations will be provided separately in the pull request

It was added the date of this update