MAINT: Separate volume and direction while calculating volume share.

In the volume share slippgae, the current amount had the direction,
i.e. buy/sell was baked into the value of `cur_amount` which then
needed to have the direction multiplied out when parts of the slippage
model needed to take in just the magnitude/amount into account.

So calculate the current volume, use that in calculations and then
apply at the order direction at order time.

Also, when applying the direction to the magnitude of the sell,
use copysign to make the code more explicit about taking the direction
of the order, instead of it possibly having some scalar impact.

As well as remove direction from volume_share calculation since that
calculation and subsequent calculations only care about the magnitude,
so make the disregard for direction more explicit by removing it.
MAINT: Factor out order processing from slippage models.

Step towards having a shared simulate method in the base SlippageModel,
which would call a process_order method in each model.
ENH: Provide a process_order method to override for custom slippage.

To make implementing a custom slippage model more straightforward,
provide a simulate method that will setup the calling of
`process_order`, which individual slippage models override to
do the unique slippage handling, where the simulate method handles
the boilerplate of checking order triggers, etc.
DEV: Don't invoke hardwired Python

Instead use /urs/bin/env to detect the Python interpreter. This way the
scripts work better with the possible virtual environment.
DOC: Fix docstring for TradingAlgorithm

Fix the example given in the class docstring as the original didn't work
on REPL.
DOC: Fix docstring for TradingAlgorithm

Get the amount from the algorithm object instead of referring to
a (undefined) global.
Merge fixes to docstring for TradingAlgorithm.
DEV: Make the other example charts larger as well
DOC: Reference example in quickstart to keep it up-to-date.

Fix the quickstart docs by including the example so that it's
always up-to-date.