Improved documentation and inclusion of set_sig_figs() in imports

This release includes three updates:

• aguaclara.core.pipes is now included in the AguaClara package documentation
• The documentation site homepage has instructions on installing, contributing to, and using the package
• aguaclara.core.units has been added to package level imports, so set_sig_figs() can be accessed like so:

  import aguaclara as ac
  ac.set_sig_figs(3)