getDouble method
Load the enviroment variable value as a double
If variable with name
does not exist then fallback
will be used.
However if also no fallback
is supplied an error will occur.
Furthermore an FormatException will be thrown if the variable with name
exists but can not be parsed as a double.
Implementation
double getDouble(String name, {double? fallback}) {
final value = maybeGet(name);
assert(value != null || fallback != null,
'A non-null fallback is required for missing entries');
return value != null ? double.parse(value) : fallback!;
}