Formulas which return a string value are quoted

[jeromesimeon]

Describe the bug
When drafting a template containing formulas, if the value of the formula is of type string, it is quoted unnecessarily.

To Reproduce
Steps to reproduce the behavior:

1. Go to the template library to the fixed-interests template
2. Parse to get the data: cicero parse --output data.json
3. Draft to see the formula value: cicero draft
4. See error:

bash-3.2$ cicero draft 
11:39:10 AM - info: Using current directory as template folder
11:39:10 AM - info: Loading a default data.json file.
11:39:12 AM - info: Fixed rate loan
----

This is a *fixed interest* loan to the amount of £100,000.00
at the yearly interest rate of 2.5%
with a loan term of 15,
and monthly payments of {{%"£667.00"%}}

Expected behavior
The same but without quotes in the formula:

bash-3.2$ cicero draft 
11:39:10 AM - info: Using current directory as template folder
11:39:10 AM - info: Loading a default data.json file.
11:39:12 AM - info: Fixed rate loan
----

This is a *fixed interest* loan to the amount of £100,000.00
at the yearly interest rate of 2.5%
with a loan term of 15,
and monthly payments of {{%£667.00%}}

Additional context

Related to issue #768

[jeromesimeon]

Fixed in #776