[FEATURE] - Validate Input Structures

[coltonbh]

I ran this command:

create-mep --recursive --minimize-ends ./rc-opt-crest-ch2cl2.xyz ./int6.xyz

And got this error

ValueError: setting an array element with a sequence. The requested array has an inhomogeneous shape after 1 dimensions. The detected shape was (2,) + 
inhomogeneous part.

The issue turned out to be that the start and end structures had different numbers of atoms. It could be nice to perform an initial validation on the structures and then explicitly tell users what issues need correcting.