Update based on changes from derived class

[tpersson]

This PR does two things:

1. Adds the option (OPTION, update_from_parent) which changes the behavior of the update an element. If the option is enabled any element derived from another class will also be updated in case the parent class is changed. This feature was requested by the FCC study.
2. Adds a warning in case you define a multipole with non-zero length.

[tpersson]

This fixes the issues #731 and #723.

[coveralls]

Coverage increased (+0.01%) to 54.249% when pulling 4450637 on tpersson:update_parent into d2684a2 on MethodicalAcceleratorDesign:master.

[madcern]

This PR does not change any default behavior and is therefore rather safe.