clean-up the reading and processing of input data

[bstabler]

It would be good to clean-up the reading and processing of input data (i.e. the default namespace) now that pipelining is implemented. This means:

• creating new model steps for loading and processing inputs
• making the default tables just tables with key columns
• moving all the orca computed columns to a mtctm1 folder/namespace for example

We want to change the meaning of

from activitysim import defaults

to be generic

[bstabler]

see #134, #137 as well

[bstabler]

We're moving away from computed columns and toward model pre- and post-processor table annotations. See the documentation for more information.