Add support for arbitrary precision computations

[albertomercurio]

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Description

In this PR, I add the support for arbitrary precision computations, for example using arrays of BigFloat type or similar. This allows to do simulations when values smaller than $10^{-14}$ are involved in the computation.

I have tested it to the following use cases:

• Schrodinger equation (sesolve)
• Master equation (mesolve)
• Sparse eigenvalue problem (eigsolve(...) or eigenstates(A, sparse=true...))

For the eigenvalues, the user need to import a package that defines a method for te following functions

• schur!
• ordschur!

I found GenericSchur.jl to work well, even without writing an extension.

In the next PRs I will write some documentation, as I think it is a very important feature that other packages don't have.

[ytdHuang]

LGTM!