Subscript Lag operator indices

To highlight that we are not multiplying the lag operator with
constants, rewrite the indices as a subscript so that are identified
clearly as such.
Merge pull request #1 from blue-yonder/L-index

Subscript Lag operator indices
FIX: Updated Travis build
ENH: Update to PyScaffold 1.3.1 of PyDSE
FIX: Rollback to conda for unit tests
DOC: Updated docs
FIX: Set min numpy version to 1.7
DOC: Small fixes
FIX: Add package data again
Add pretroleum notebook and exercise