Increase speed of get_custom_effort()

[damianooldoni]

This issue is a spin off of #333. It seems that get_custom_effort2.zip of @MartijnUH is faster than get_custom_effort().
How much fast? Is it worth adding other dependencies (e.g. reshape)?
A benchmark is maybe needed.

However, as we are now focussing on releasing camtaptor v1.0.0, this is not a priority at the moment.