TRestDataSet new strategies 'append' and 'extend' for RelevantQuantity

[AlvaroEzq]

Adding new strategy options for RelevantQuantity.

• append: It will append the metadata member value to the previous value. The user can specify the appender string after the append keyword. If no appender is given, then an empty string will be used. It will check if the value is already in the string and not append it if it is. Use extend if you want to force the append.
• extend: same as append but it will not check if the value is already in the string.

These are useful for example for the relevant quantity containing the run number metadata of a TRestDataSet originated by several runs. Also for the parent run number.