wrap the pandas import

In other places, like core/topology.py, the pandas import is wrapped. This makes it possible to use features that don't require it without having it installed. Also, when it's importe you get a nice

```
The code at mol2.py:150 requires the "pandas" package, which is
an open source, BSD-licensed library providing high-performance, easy-to-use
data structures and data analysis tools for the Python programming language.

pandas can be downloaded from https://pypi.python.org/pypi/pandas or installed
with the python "easy_install" or "pip" package managers sing:

# easy_install pandas
or
# pip install pandas
```

when it's done this way