fixed str to int bug

[philsippl]

For some obj files the import failed with following error:
TypeError: unsupported operand type(s) for -: 'str' and 'int'

This was due to this line here: mesh -= 1 where in some cases the pandas failed to convert to int, but instead converted it to string.
However, an float array can't be used as an array indexer, so the astype conversion is necessary.
I think this is also related to #230

[daavoo]

Hola @philsippl ! Sorry for late reply and thanks for the P.R. I think that this has been already fixed in #253
Mant thanks again tough