Euphonic: Resolution Library

[rebeccafair]

Many codes (e.g. Euphonic, Abins) want to be able to apply instrumental resolution functions to their data that are more realistic than just Gaussian broadening, but less complex than the full resolution convolution offered in Horace. Rather than embed this functionality in a specific code, it could be provided in a separate library so it can be used by and offer benefits to many different codes.

Potential tasks:

• Consider further broadening speed-up by modern image-processing techniques #176
• Instrumental broadening #175