Add support for inserting population by percentage instead of number

[cainja]

Current patch action PatchActionInsert inserts cells based on a single number. For example:

<action.parameter id="INSERT_NUMBER" value="N" />
will initialize N cells for the population.

Proposal:

Update PatchSeries to additionally parse configs in the form:
<action.parameter id="INSERT_PERCENT" value="N" />
or
<action.parameter id="INSERT_NUMBER" value="N" />
to mimic behavior added in #53.

This issue is similar to #49.
Adding this feature is necessary for #21.