Issue 334 derivative

[adler-j]

• Made diagnostics in general less verbose.
• Improvements to derivative test, should close Derivative test needs some rethinking #334
• Moved sample generation to set, can now be arbitrarily customized. Closes A better way of generating samples from a set #67
• Added PowerOperator (does what you'd expect), needed for example.

[kohr-h]

""" on wrong line

[kohr-h]

"By default, the generator yields ..."

[kohr-h]

Looking good in general. Just some minor remarks.

[adler-j]

Fixed most comments, two bigger style changes

• np.array([1.0]) is no longer a valid input array, it caused several bugs (it was cast to a scalar, which threw an error). It should likely be interpreted as a scalar.
• Moved discretization examples to fspace, now simply discretizes the uspace examples.

Also minor style, for example update the examples file.

[kohr-h]

Good to go