compilation warnings

[yurivict]

clang-8 prints these warnings:

/wrkdirs/usr/ports/science/chrono/work/chrono-5.0.0/src/chrono/physics/ChContactSMC.h:230:29: warning: enumeration value 'Perko' not handled in switch [-Wswitch]
                    switch (adhesion_model) {
                            ^
/wrkdirs/usr/ports/science/chrono/work/chrono-5.0.0/src/chrono/physics/ChContactSMC.h:165:17: warning: enumeration value 'Flores' not handled in switch [-Wswitch]
        switch (contact_model) {
                ^
/wrkdirs/usr/ports/science/chrono/work/chrono-5.0.0/src/chrono/physics/ChContactSMC.h:272:17: warning: enumeration value 'Perko' not handled in switch [-Wswitch]
        switch (adhesion_model) {
                ^

[rserban]

Indeed, these are new models that are currently only implemented in Chrono::Parallel. They will eventually be also supported for the base Chrono SMC system.