Rename the first positional arg in _trapz to match numpy

[varchasgopalaswamy]

As implemented currently, the trapz function is not consistent between numpy and pint

x = np.linspace(0,1)
x2 = x * unit_registry('m')
y = x**2 
y2 = x2 ** 2

np.trapz(x=x,y=y) # Works 

np.trapz(x=x2,y=y2) # Throws error 

The reason is that as in the numpy documentation, the first positional argument for trapz is y, while pint's overriding implementation has it as a, which is more like the unary numpy functions.

[dopplershift]

Ooops, I missed the need to match for keyword-based calls.

[codspeed-hq]

CodSpeed Performance Report

Merging #1796 will not alter performance

_{Comparing varchasgopalaswamy:master (aa5d7a0) with master (6c2dda9)}

Summary

✅ 421 untouched benchmarks

[hgrecco]

Thanks!