Option to not expand elements to their constituent isotopes

[alberto743]

By default, the converter employs the openmc.Element.expand method to decompose the natural isotopes, i.e. those having the atomic mass number equal to 0.
This behavior lets the materials generated for OpenMC does not match the corresponding ones defined in the MCNP input deck.

It is worth mentioning that the natural isotopic abundances may be defined according to different sources, e.g. NIST or IUPAC, or selecting either the center of the range provided or the best measurement.
Indeed, depending on the desired nuclear data libraries and the isotopes defined therein, the analyst may wish to keep the original isotope definition as in the MCNP input deck.

This branch provides an additional command-line switch that allows the user to unselect the automatic expansion of natural elements according to the abundances encoded in the OpenMC sources so that the materials defined are 1:1 with respect to the definitions available in the MCNP input deck.