Multiplier for the entire population?

[BoPeng]

We allow syntax such as

--popsize A=100 B=200 --symptomatic-r0 A=2

to set r0 to be twice as the default value. However, we do not allow the multiplier to be set for the entire population, with syntax similar to

--symptomatic-r0 ''=2

or simply

--symptomatic-r0 =2

although the later looks strange.

[BoPeng]

all=1.2 is allowed, which is clearer than `=1.2'