Restore instruments serialization (#512)

Serialization is used for copying instruments into the new job; broken serialization ---> sumulation is done for the default instrument parameters.

Also to avoid multiple inheritance in instrument items the following changes were made:

ScanningItem was renamed to ScanningInstrumentItem and inherits from InstrumentItem.

SpecularInstrumentItem, DepthprobeInstrumentItem and OffspecInstrumentItem inherit from ScanningInstrumentItem

This resolves #512 (closed)