Error on invalid LAMMPS instance

[Joroks]

I've taken a few things from #34 and made sure that we check the lammps instance before using it for all of our public API

[vchuravy]

Ah interesting, so the issue with checking before an operation is that the error is "far" away from the original location.
Once should always check after so that the error is thrown close to the offending operation.
It's probably sensible to check before as well to make sure we don't pass an invalid state.

[Joroks]

The main issue I'm adressing here is that it's not valid to pass null pointers as the handle for a lammps instance (with very few exceptions). Doing this currently causes a crash. I've decided to include this check in the check method for simplicity. We could also seperate that out though.

[vchuravy]

Actually we can define next to our unsafe_convert function:

Base.cconvert(::Type{Ptr{Cvoid}}, lmp::LMP) = lmp.lmp == C_NULL ? error("") : lmp

So that whenever a LMP instance is passed to a C-API we emit an error check.