Macro for fasterMulticolvar+named modes

[Iximiel]

@gtribello
I am starting to do some work to bring Multicolvar.h to be more compatible with the GPU

This first commit just aims to clearer the "mode" variable, and add a small layer of automatism to setting up a colvar to be compatible with a Multicolvar with a few macros. I think this could make it easier to change the function signatures faster.

For running on a GPU I may need to restart implementing #1092

[Iximiel]

Update:
I simplified the calculateCV interface for the output part, this will likely make it easier to manipulate how that will work with managing the output data without changing the function signature.

I also want to do something similar for the input variables, but I have to deal with the fact that some CVs, like the dipole one, use some of the input values as temporary values, so the signature changes with some input not set as const ref but as simple refs

[Iximiel]

Update: I set up also an Input helper with a test-guide.

I am working on changing the action atominstic ptr to something similar to the input/output classes
I was thinking on setting up something similar to the mode variable
The aim of this is to set up an object simple -and light- enough to be loaded on the GPU without excessive complications

[Iximiel]

@gtribello can you look at the Quaternion::calculateCV method?

in the //convert to quaternion section, the one with all the "ifthenelseif" blocks, in the first block S is a double in the others is a float, since everything else is double precision I fear that it is a copy-paste error