Allow passing Ecut as a keyword argument

[mfherbst]

Also makes the compiler emit a deprecation warning (if explicitly turned on) about the old interface being deprecated now.

[antoine-levitt]

Ah you beat me to it, I was just doing this now

[antoine-levitt]

I had a

## Note the implicit passing of keyword arguments here:
## this is equivalent to PlaneWaveBasis(model; Ecut=Ecut, kgrid=kgrid)

in tutorial.md because that might be confusing to new julia users

[mfherbst]

Sorry ... I thought it was clear in the comment I left that I would do it.

[mfherbst]

good point

[antoine-levitt]

There's a bunch of other places we could replace the pattern X=X (or X = X) by X, but it has been too long since I used regexpes

[mfherbst]

Yeah, I know, but I suggest we just fix it when we find it.

[antoine-levitt]

Yup, just might have been a good candidate while we were search&replacing stuff