Improvements to make_region

[mtfishman]

make_region is introduced in #77 to make a region of sites that will be updated in an alternating update/sweeping algorithm. This issue is to track potential improvements, for example:

1. Generalize it to n-site updates.
2. Think about how we might improve the design of how the reverse step of TDVP is incorporated.

[emstoudenmire]

3. Split the make_region function (in tree_sweeping.jl) into separate functions, perhaps distinguished by different Val like arguments.