Add information about net & gross return

Problem description
Some key indicators for the transaction of investment properties are missing in SwissRETS.

Desired Solution
Add information about:

• gross return (actual): gross return in percentage by reporting date (net annual rental income x 100 / investment costs)
• gross return (target): targeted gross return in percentage (net annual rental income x 100 / investment costs)
• net return (actual): net return in percentage ((Net annual rental income - running costs) x 100 / investment costs)

[jensstalder]

Example maibe?

How would these numbers look like? (Don't think my examples are correct)

<property>
   <characteristics>
       <currentInvestmentGrossReturn>13.5</currentInvestmentGrossReturn>
       <targetInvestmentGrossReturn>17.5</targetInvestmentGrossReturn>
       <currentInvestmentNetReturn>17.5</currentInvestmentNetReturn>
   </characteristics>
</property>

A bit high maybe but ok in general

<property>
   <characteristics>
       <currentInvestmentGrossReturn>3.18</currentInvestmentGrossReturn>
       <targetInvestmentGrossReturn>3.23</targetInvestmentGrossReturn>
       <currentInvestmentNetReturn>2.35</currentInvestmentNetReturn>
   </characteristics>
</property>

[jensstalder]

<property>
   <characteristics>
      <grossPremium>
      <grossPremiumTarget>
      <netPremium>
   <characteristics>
</property>