check for missing masses

Moltemplate (specifically "lttree_postprocess.py") now complains whenever the user fails to specify the mass of one of the particle types in the "Data Masses" section.

(This assumes that a "Data Masses" section exits. If not, moltemplate prints a warning message. Moltemplate does not force users to define a "Data Masses" section. But if one is present, moltemplate will check that all atom type masses are defined there.)