Mark Timezone member functions as const if they don't modify member data

[Abrynos]

Member functions that do not modify any member data can be marked as const.

Example usage:

namespace whatever {
    static Timezone tz;

    const Timezone * get_time() {
        if (!ezt::waitForSync(1)) {
            return nullptr;
        }

        tz.setLocation("Europe/Vienna");
        &tz;
    }
}

void other_code() {
    const Timezone * tz = whatever::get_time();
    if (tz == nullptr) {
        return;
    }

    // previously not possible because the local variable "tz" is marked const but the method dateTime() is not
    Serial.printLn(tz->dateTime());
}

[Abrynos]

@ropg Any thoughts on this?