fix OperatorAdapter.__repr__ index out of range

See merge request ift/nifty-dev!111

Fix pickle

See merge request ift/nifty-dev!112

self._trafo \elem {1,2,3}. Therefore, either list needs to have
four items or should be accessed via [self._trafo - 1].
This way seem more readable to me.

Bug can be triggered by creating a FFTOperator `op` and calling
`print(op.inverse.adjoint)`.

Add svg as allowed output format for nifty plots

See merge request ift/nifty-dev!110

choose mutifield key

See merge request ift/nifty-dev!108

Resolve "Questions on SymmetrizingOperator"

Closes #52

See merge request ift/nifty-dev!105

Remove exp from multifrequency correlated field

See merge request ift/nifty-dev!107

Check inputs for operators

See merge request ift/nifty-dev!104

Related to Issue #62

allow specification of constants for samples

See merge request ift/nifty-dev!103

Remove MultiField.update()

See merge request ift/nifty-dev!102

The reason for this is the following. MultiFields behave very similar to
dictionaries except for the fact that dictionaries are mutable and MultiFields
are immutable. Therefore, MultiField.update() needs to return a new object. That
is different to the dictionary.update() functionality and therefore unintuitive.

MultiField.union() can do the exact same thing as update() used to do.

Cleanup

See merge request ift/nifty-dev!101