Create QN FAQ section and document sysimages for Jupyter and VS Code [no ci]

Author: emstoudenmire

docs/settings.jl

@@ -46,6 +46,7 @@ settings = Dict(
     "Frequently Asked Questions" => [
       "Programming Language (Julia, C++, ...) FAQs" => "faq/JuliaAndCpp.md",
       "DMRG FAQs" => "faq/DMRG.md",
+      "Quantum Number (QN) FAQs" => "faq/QN.md",
       "ITensor Development FAQs" => "faq/Development.md",
       "Relationship of ITensor to other tensor libraries FAQs" => "faq/RelationshipToOtherLibraries.md",
       "Julia Package Manager FAQs" => "faq/JuliaPkg.md",

docs/src/faq/QN.md

@@ -0,0 +1,24 @@
+# Quantum Number Frequently Asked Questions
+
+## Can I mix different types of quantum numbers within the same system?
+
+Yes, you can freely mix quantum numbers (QNs) of different types. For example,
+you can make the sites of your systems alternate between sites carrying
+spin "Sz" QNs and fermion sites carrying particle number "Nf" QNs. The QNs will
+not mix with each other and will separately be conserved to the original value
+you set for your initial wavefunction.
+
+## How can I separately conserve QNs which have the same name?
+
+If you have two physically distinct types of sites, such as "Qudit" sites, but
+which carry identically named QNs called "Number", and you want the qudit number
+to be separately conserved within each type of site, 
+you must make the QN names different for the two types of sites.
+
+For example, the following line of code will make an array of site indices with the qudit number QN having the name "Number\_odd" on odd sites and "Number\_even" on even sites:
+```
+sites = [isodd(n) ? siteind("Qudit", n; dim=10, conserve_qns=true, qnname_number="Number_odd") 
+                  : siteind("Qudit", n; dim=2, conserve_qns=true, qnname_number="Number_even") 
+                  for n in 1:2*L]
+```
+(You may have to collapse the above code into a single line for it to run properly.)

docs/src/getting_started/RunningCodes.md

@@ -102,3 +102,23 @@ alias julia_itensors="julia --sysimage ~/.julia/sysimages/sys_itensors.so -e \"u
 ```
 
 where of course you can use the command name you like when defining the alias. Now running commands like `julia_itensors code.jl` or `julia_itensors` to start an interactive session will have the ITensor system image pre-loaded and you will notice significantly faster startup times. The arguments `-e \"using ITensors\" -i` make it so that running `julia_itensors` also loads the ITensor library as soon as Julia starts up, so that you don't have to type `using ITensors` every time.
+
+## Using a Compiled Sysimage in Jupyter or VS Code
+
+If you have compiled a sysimage for ITensor as shown above, you can use it in Jupyter by running the following code:
+```
+using IJulia
+installkernel("julia_ITensors","--sysimage=~/.julia/sysimages/sys_itensors.so")
+```
+in the Julia REPL (Julia console).
+
+
+To load the ITensor sysimage in VS Code, you can add 
+```
+"--sysimage ~/.julia/sysimages/sys_itensors.so"
+```
+as an argument under the `julia.additionalArgs` setting in your Settings.json file.
+
+For more information on the above, see the following [Julia Discourse post](https://discourse.julialang.org/t/using-an-itensors-sysimage-when-starting-the-julia-repl-in-vs-code/98625/4).
+
+