Make `nupropagator` an optional dependency

Closes #19 (closed) Now the default installation

pip install ntsim #from PIPy

will install without the nupropagator dependency.

If the user wants it, they can specify:

pip install ntsim[nupropagator]

Similarly, when you want to install your local version of the package:

git clone git@git.jinr.ru:dnaumov/ntsim.git
cd ntsim
#default install:
pip install .
#specific install with nupropagator dependency:
pip install ".[nupropagator]"