MyInstrument edits

1. Introduced val_list property for the commands. For those commands, which val_list is non-empty, the value is checked by the commandParser to be within val_list each time writeProperty is called
2. AttributesFromFormatSpec is slightly generalized, now it is capable in principle to hadle numbers between % and format specifier, e.g. '%2.5e'
3. Changed behavior of 'readProperty' in the case when no format specifier is given for a read-only command. No no processing is done on the returned string.