Add parser for printed scalars output

[ivandi69]

In custom simulations one can print measurement or calculation results.
For example

ac lin 100 1meg 10meg
write custom.plot v(out)
meas ac fcenter max_at v(out)
let vmax=vecmax(v(out))
print fcenter vmax > custom.print

The custom.print file looks like this:

fcenter = 1.234e+01
vmax = 5.678e+00,4.321e-01

The parser adds printed results to the dataset as scalars.

[ra3xdh]

Thanks for the contribution! This addition will help to implement the .MEASURE support and the saveOp option #124