Defaults for most/all parameters in configuration

[jgbarah]

For most (maybe all) of the parameters specified in a SirMordred configuration file, a default value could be provided, so that the minimum configuration files (if default values are suitable) are much more simple.

There are many ways of implementing this, but one of them could be just having a default configuration file with the default options, and ensuring it is loaded before the user-provided configuration files. This is easy to do, in the loop that loads user-provided config files. I can propose a pull request, if this approach seems reasonable. A side effect of this would be having a complete or semi-complete configuration file with default values, that could be used as an example.