fix: specify all parameters for DemandFlexibility struct

[lanesmith]

Purpose

The purpose of this code is to fix the error discussed in #129. When a demand_flexibility.csv file is not available, not all of the parameters of the DemandFlexibility struct were defined.

What is the code doing?

In the try/catch loop that is responsible for trying to load the demand_flexibility.csv file in read_demand_flexibility, the three parameters that were previously unspecified are now specified.

Where to look

The fix lives in the read_demand_flexibility function in src/read.jl.

Testing

I tested the fix on my local machine and the error presented in #129 no longer occurs. As @BainanXia mentioned, the demand flexibility functionality is not yet integrated into PowerSimData, so I cannot be sure that there are not any errors on that end. I also have not tested this with plug, which is where the error was first noted. However, based on the error reported in #129, I believe this fix should do the trick.

Time estimate

This review should be pretty quick.

[BainanXia]

Looks good to me. Maybe @jon-hagg could give a quick try.

[jenhagg]

Works for me, thanks!

[lanesmith]

Great, I'll merge it in!