Non-float data types

[SJSeckmeyer]

Do we want to handle all operations (boolean comparison, etc.) in our arrays a FFT of an array of booleans does not really make sense. But we still want those arrays to have named dimensions.
Do we want to work with other libraries (see #38) or support this? How far should this support go?

[SJSeckmeyer]

Discussion Result: Allow also int and bool in order to not have to interact with other named array libraries. For now support it by keeping data types as much like numpy as possible on operations. We will not support arbitrary indexing with a boolean array because that would not have a well-defined FFTDimension as a result. Unless it becomes too much of a hassle or impedes other designs. Then we may revisit this.

Will be tested in the implementation of #54.