fix collection option setting

The previous version would not allow you to set a default value correctly through the XML file. This arose since the default optimisations were set up in a map which was not cleared when a new default option was requested in the xml file. This has now been fixed by introducing a clear to this map

In addition FieldConvert was using too much memory by calling the Collection operators which are designed to be efficient when using a large number of cores and so the additional memory requirements are not too onerous. This has been addressed by adding another default argument into ExpList constructors so that the eNoCollection option can be called from the FieldConvert intialisation of ExpList.