Make Pandas dependency explicit

[carschno]

This project depends on Pandas. However, it does not explicitly declare the Pandas dependency. Instead, it relies on Pandas depencies declared in the pagexml-tools dependency.

pagexml-tools v0.6 drops the Pandas dependency, see #220 .

• investigate dependency on Pandas (do we need it at runtime or for testing only? could we get rid of it entirely?)
• add explicit Pandas dependency as far as necessary