Make the macros module into a subpackage

[blakeaw]

Maybe it would be cleaner or easier to understand if the macros were organized into their own type-specific modules. Something like macros.pk, macros.pd, and macros.dose for PK, PD, and dosing functions respectively.

In this case, the current macros module under pysb.pkpd.macros would be converted into a sub-package. The modules inside that sub-package could then be imported in the pkpd level __init__.py to give a shortcut where they could be imported like from pysb.pkpd import pk, pd, dose. Then users could call those like pk.clearance(...), pd.emax(...), or dose.bolus(...) when building their own model, which may make it more readable and easier to differentiate the PD and dose functions from the PK ones.

[blakeaw]

Using pd for PD macros could potentially cause clashes with pandas, which is usually shortened to pd during import: import pandas as pd. This is something else to keep in mind. However, most users won't use pandas when encoding a model, so it probably won't be a big issue.