Allowing custom grid definition for each bias instance

[giacomofiorin]

See #468 and links therein.

The following is a list of classes that use grids, and which could be optionally be allowed to define custom grids:

• [ ] colvarbias_abf (#710)
• [x] colvarbias_histogram
• [ ] colvarbias_meta (#733)
• [ ] colvarbias_opes (#733)
• [ ] colvarbias_ti (#733)
• [ ] colvarbias_reweightaMD (#733)

[giacomofiorin]

Adopt from #342 the keyword grid as the standard keyword for biases.