Replicate pulseq functions to create sequences

[beorostica]

We already have functionality for creating events similar to pulseq functions, for example:

% Matlab Pulseq Code
seq = mr.Sequence()
rf = mr.makeBlockPulse(args...)
gr = mr.makeTrapezoid(args...)
adc = mr.makeAdc(args...)

# Julia KomaMRI Code
seq = Sequence()
rf = RF(args...)
gr = Grad(args...)
adc = ADC(args...)

However, we don't have all functions for combining events. In matlab pulseq this is done the addBlock() function:

...
seq.addBlock(rf, gr, adc)

The analog combination of event and concatenation in a sequence would be something like this:

...
seq += (rf, gr, adc)

There are many options to be aware when dealing with combination of events, for example we could even sum two gx gradients.

[cncastillo]

This isssue is the same as #4.