MAINT: centralize dataframe schemas

[wasade]

In support of #978, dataframe schemas should be centralized into a separate module. We should consider whether to allow to/from methods to inherit the dtype of axis labels (constrained to int and str). If Table can support int and str as a label type, then the transformation is easy.

The reason to support int and str is that, in general, labels originating from qiime2 are str. However, use of Table within a data management platform would benefit from simpler types for any upstream relations, and therefore should be allowed.