Add the ability to set ncrit for xfoil calculations

[JeanMRog]

ncrit is a key parameter of xfoil it is important to be able to set it, the default value of 9 is not always the most adequate setting depending of the usage. gliders will look at values of 10 and above foil in water will look at values around 3.

[inse0918]

add this lines after defining mach number line like below screenshot

    # ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
    #    Defining N-crit Number in VPAR
    # ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
    # issueCmd('MACH {!s}'.format(Mach))
    # Defining the N-Crit Number in Viscous Setting
    issueCmd('VPAR')
    issueCmd('n')
    # Defining N-Crit number
    issueCmd('%f' % Ncrit)
    issueCmd('')