Adaptive linear bias 

This pull request is for an implementation of an adaptive linear biasing technique called "Experiment Directed simulation" described in [this paper](http://dx.doi.org/10.1021/ct500320c). There are 2 main changes to the code. The first is refactoring `colvarbias_harmonic` to a `colvarbias_restraint`, which allows a linear and harmonic version of the restraint class. The second change is the addition of an adaptive linear bias method. I've also added the corresponding documentation and references.

I'm happy to make any changes to the code to make it easier to pull. Also, thank you guys for developing colvars which made my research much easier.