More flexible bias-specific grid parameters

[jhenin]

I think histogramGrid is useful, and I would like to make it easier to use, and adopt it for ABF (and potentially metaD as well). A plan for that:

1. change the f_cv_grid feature - potentially remove it entirely, as the grid is not really a feature of the cv anymore. In particular, the requirement for boundaries at the colvar level are moot if grid parameters are given elsewhere
2. rename histogramGrid to grid so the same keyword can be used within different biases and documented once
3. Implement it in ABF

[giacomofiorin]

All are okay with me.