Add a ramp function generator in event scheduler.

Add a ramp function generator in event scheduler.

So far, it is possible to schedule changes to link parameters
(bandwidth, delay, BER, duplicate) and attributes (width, color) to
constant values or periodic functions (rand or ramp).

Example:

link l1 {
    nodes {n1 n2}
    bandwidth 256000
    events {
        1 width ramp 5 2 1
        3 color rand 0 16777216 1
        5 bandwidth rand 128000 256000 2
        30 width 3
        35 color red
    }
}

At t = 1s after the experiment is started, the width attribute of link l1
will be set to 5, and will increase by 2 each second, until t = 30s when
it will be set to constant value of 3.

At t = 3s the color attribute of link l1 will randomly change each second,
until t = 35s when the color will be set to "red".

Ar t = 5s bandwidth will begin to take random values between 128000 and
256000 each 2 seconds, and this process will continue for the entire
duration of the experiment.

All changes trigged by the event scheduled will be visible in the GUI.

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