W values and Fano factors

• Implement default values for W and F for all available gases.
• By default, TrackSrim and TrackTrim now use the W value and Fano factor provided by the Medium. The possibility to set W and F explicitly in TrackSrim and TrackTrim is kept for backwards compatibility for the time being.
• By default, TrackSrim now retrieves the atomic number of the target and the number density from the Medium (i. e. the effective A and Z of the target do not need to be set explicitly by the user although this is still possible).

mentioned in commit 140a3ef1

merged