Making null operators implicit

[aubertc]

This issue is two-fold:

• The program should be able to parse "0" (or any other way of denoting the null process) as the null process,
• The null process should be kept implicit if wanted by the user.

That is, both a.0 and a should be interpreted as the prefix a followed by the null process.

[aubertc]

cf. https://github.com/peterbro1/RCCS_Impl/commit/5d596329eee439f9e710aee884ccf679ef34cfd5#r68487236

[peterbro1]

Fixed in commit https://github.com/peterbro1/RCCS_Impl/commit/ef515370562014c51042011fd458fcdc46a4a7ae