add logistic sigmoid transformation and corresponding test

Address bug https://bugs.launchpad.net/oq-svir/+bug/1380978
Add logistic sigmoid function to the set of available attribute transformations, together with the corresponding test.
Add the inverse function as requested by the updated bug description.
Produce Null output values when unacceptable inputs are found, and display the list of invalid inputs to the user in the QGIS notification bar.