Add dynamic power reactor

[nsryan2]

Summary of changes

DPR is a Cyclus reactor archetype based on Cycamore's Reactor, except that it has the ability to update the power output over time.

Outside of refueling, the power output of the reactor can be modified by a percentage in the Tock phase of each time step. This feature is optional, and will default to 100% of the power_cap variable the user identifies. At each time step, the reactor will progress through the percentages; as such, there must be a percentage for each time step (if the length of the power_percent_list is shorter than the number of time steps, the reactor will default to 100%).

To simulate 10 time steps, a user could provide a list like:

<power_percent_list>
    <val>0.3</val>
    <val>1</val>
    <val>1</val>
    <val>1</val>
    <val>1</val>
    <val>1</val>
    <val>0.9</val>
    <val>0.5</val>
    <val>1</val>
    <val>1</val>
</power_percent_list>

If the simulation was extended to 11 time steps with out the list changing, the 11th time step would have a power output of 1 (or 100%).

Types of changes

• Bug fix (non-breaking change which fixes an issue)
• New feature (non-breaking change which adds functionality)
• Breaking change (fix or feature that would cause existing functionality to change)
• I have read the CONTRIBUTING document.
• My code follows the code style of this project.
• My change requires a change to the documentation.
• I have updated the documentation accordingly.
• I have added tests to cover my changes.
• All new and existing tests passed.

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