Change the salt concentration definition & make negative ion dia

[nasimanousheh]

@jadhao @kadupitiya

• I changed the salt concentration definition based on positive ions.
• The negative ion diameter is now a variable that user can inter the value.
• I tested the simulation (for 1ns), for 0.714_0.714_1-1_0.5_3 and 0.714_0.357_1-1_0.5_3. The output has not changed comparing to master.
• I also did the simulation for 0.357_0.714_1-1_0.5_3 and compare it with 0.714_0.357_1-1_0.5_3. There is a slight difference between them, because the unit length has changed (it is equal to positive ion diameter). So, the bin size will change according to unit length.

[jadhao]

@nasimanousheh Great! Approving. @nasimanousheh @kadupitiya Please do a re-install and test it on the Jupyter platform with new widgets to see if asymmetry is correctly incorporated and results look reasonable off the gui