reduce and reuse maz to maz network impedance calls to speed up runtime

[bstabler]

See #431. We'll want to pull in these example expression file improvements once ready.

[coveralls]

Coverage increased (+7.1%) to 63.587% when pulling 015dcb8 on fewer-maz-maz-data-calls-for-sandag-two-zone into ca2edd1 on develop.

[coveralls]

Coverage increased (+6.4%) to 65.266% when pulling 48e25df on fewer-maz-maz-data-calls-for-sandag-two-zone into 6ba7e8f on develop.

[esanchez01]

@bstabler, @wusun2 and I are interested in knowing whether this change improves the 1 and 3 zone runtimes as well. I tested the 1 zone system on our machine but the runtimes varied quite heavily from run to run. From our past discussions/shared runtimes, your machine is more consistent -- have you tested the 1 and 3 zone systems on your machine?

[esanchez01]

@bstabler, per our discussion, I applied these changes to the 1 and 3 zone setups as well. You can find the updated setups here for testing: https://github.com/esanchez01/activitysim/tree/fewer-maz-maz-data-calls-for-sandag-two-zone

[bstabler]

thanks @esanchez01, I merged your revisions