Bug/issue#20 (#123)

This PR addresses two issues that were closely related:
* correctly accounting for the current contribution of density mechanisms to CVs at branch points, where the density mechnanism is not present on all branches. This was discussed in issue #20.
* adding support for weighting of current densities calculated from density mechanism. This is required to weight the current contribution to CVs in issue #20.

## small updates

* update CMake rules for finding libunwind, because it broke for some reason.
* add `binary_find` and unit tests to the algorithms library
    * returns an iterator, as opposed to `std::binary_search`, which returns a boolean.
    * works with ranges.
* added `subrange_view` specialization that takes a subrange specified by a pair of indexes
* added `assign_from` to range utils
    * a helper function that returns a proxy type that can be copied into a container
    * evaluate a range and store contents in a container, with minimal verbosity in user cod...