add SubDitStr function to enable DitStr slicing

[hz-xiaxz]

I studied the official implementation of SubStrings in Julia's Base library SubString.jl

I think same implementation can be added into BitBasis.jl for high performance slicing, so I imitated the implementation in the Base Library in this PR

[hz-xiaxz]

This is related to (#53)

[hz-xiaxz]

Sorry that I may have accidentally modified some other lines with my auto formatting tool :(

[GiggleLiu]

Thank you for the contribution! The code quality is pretty good. I am wondering if there is a practical using case for this feature?

[hz-xiaxz]

I am using LongBitStr to represent the basis vector in Fock space. In spinful Fermion systems, as hopping is usually not allowed between spin up and spin down fermions, we can divide the whole Hilbert space into tensor products of spin up and spin down Hilbert spaces, i.e. $ V = V_{up}\otimes V_{down} $ . Thus, the basis vector can be stored as $ |0101>\otimes |1010> $ or simply $ |01011010> $. In Variational Monte Carlo simulation, some updates (like the change of Slater Determinant) only relate to the subspace vector, and some other updates like Gutzwiller ansatz relate to the whole vector.

Hence, a SubDitStr is useful when we need to do calculation in the subspace of the whole Hilbert Space and this slicing reduces computational complexity.

The former implementation of slicing relys on the getindex method of DitStr and returns a array, it will cost redundant time to convert a array back to DitStr. This is a bottleneck in my current code.

[hz-xiaxz]

This PR is still WIP, just push here to see if it is a suitable solution.

[GiggleLiu]

This feature seems to be useful, and is throughly tested. I am wondering if it is beneficial to add some eye candies like bit_string[start:stop]?

[hz-xiaxz]

This is the last commit for this PR. If the code is approved, I hope it can be merged with a version bump. Thanks a lot for your review and kind help!

[hz-xiaxz]

Will this commit be approved now?

[GiggleLiu]

I just merged this PR and will tag a new patch version later.

[hz-xiaxz]

Thanks!