allow parsing of -nan in to<>() converter function
new to<bool>() specialisation to allow true/false, yes/no, or integer parsing
File::Config now relies on to<bool>() specialisation
new type_bool() command-line argument type
b-value scaling now defaults to 'on'

There is one command-line option to change this (-bvalue_scaling), along
with a corresponding config file entry (BValueScaling) to change the
default.