Fix numeric type arithmetics

[greschd]

Fixes #596.

The two decorators _left_operator and _right_operator handle getting the .value and checks for left-side and right-side operator, respectively.

For the arithmetic itself, the base Python types are used, and then converted back into AiiDA types using to_aiida_type. This function uses singledispatch to select the right function corresponding to the type of the value.

I removed the inline operations (__iadd__ etc.) because that doesn't really go well if the type of the return value could be changed. It would be possible to implement, but IMO doesn't give much benefit. The current version is consistent with the behavior of the corresponding Python types, which are immutable. The operations (+= etc.) can still be used, since Python just falls back on __add__.

Another detail is that the comparisons now return Bool, not bool. This should be fine since the Bool now converts to bool correctly, but it could be changed back if needed.

Side note: I think that to_aiida_type should also be used to automatically infer at least the basic types when passing arguments to processes. Having to explicitly call Str, Int etc. on all inputs is just unnecessary clutter.