Nonmonotone variant of linesearch methods

[aldma]

This PR adds the possibility to run algorithms with a linesearch procedure in a nonmonotone variant. In particular,

• this feature addresses algorithms such as PANOC, PANOCplus, ZeroFPR and DRLS;
• a kwarg monotonicity is added to control the level of monotonicity required, with 0 < monotonicity <= 1;
• the default monotonicity = 1 coincides with the current implementation (monotone descent);
• a scalar value merit is included in the state, monitoring progress instead of objective or envelope values.

[lostella]

@aldma thanks, I should be able to review this over the weekend.

I’m wondering about the test failures: the test dependency on AbstractOperators we could remove; regarding RecursiveArrayTools, not sure, I’ll need to look into it.

[aldma]

I hope I didn't mess things up with the rebasing. I am still learning gitology.