Reduce code duplication in DataLayouts

[charleskawczynski]

This PR reduces code duplication in DataLayouts, and defines get_Ni and get_Nj to allow for distinguishing from get_Nij.

This is basically a peel off from #1955.

[Sbozzolo]

This commit is a little hard to parse because lots of lines have changed, but the important changes are only a handful.
As far as I can tell, it does three things:

1. adds get_Ni, get_Nj
2. changes slab for IJFJ and for DataSlab1D, and column for VF and VIFH
3. replaces ASCII symbols with unicode symbols

Splitting off unrelated changes to different commits (in the same PR) would help with reviewing and would make git log/blame more useful.

Tests are failing, but it doesn't seem anything fundamentally wrong.

(You might also have a look at fonts with programming ligatures (like Fira Code), if you haven't already. In my opinion, they provide the best solution for symbols like ≤: when you type <= the combination of characters is rendered as ≤, but the characters remain ASCII. This ensures that the symbol is still easily to manipulate in places where it is not as trivial to enter unicode symbols (such as the command line, or the search bar in a browser), while improving readability in your editor)

[charleskawczynski]

Sounds good, yes, I can split up the commits.

[charleskawczynski]

I think some of the errors actually were related to get_Ni /get_Nj, so I removed that. Also, I removed the ≤-related changes for now. I'll take a look at Fira Code. So, this PR now only changes some of the method definitions in DataLayouts to forward to equivalent methods.