BF: make_trapezoid with amplitude + duration as inputs.

[wtclarke]

This should fix the issue described in #145. It also adds partial tests for the make_trapezoid function. Finally it removes a unused inport that was throwing warnings with tests in the pypulseq/SAR/SAR_calc.py module.

[wtclarke]

Before this is merged, it would be great to understand exactly what the calling cases should be so that we can write some proper tests for them.

[btasdelen]

@wtclarke I checked the PR, and the use cases you documented covers pretty much all the "useful" and well-defined cases I can think of.

    The user must supply as a minimum one of the following sets:
    - area
    - amplitude and duration
    - flat_time and flat_area
    - flat_time and amplitude
    - flat_time, area and rise_time

The complexity of input parameter sets bothers me, though. It is error-prone and not really Pythonic. I know it is something inherited from the Matlab Pulseq, so I don't know how to solve it without breaking the familiarity.

Are you considering adding test cases, or should I go ahead and merge?

[wtclarke]

If you are happy with the use cases then go ahead and merge. These are all already tested for in test_make_trapezoid.py. There's still lots that should be added to that test set, but this is a start.