Provide filenames and strigns

[mads-bertelsen]

Currently filenames need two levels of ", as in filename = ""PSD.dat"".

The type of the variable is known, so if "PSD.dat" is given, the extra layer could be added automatically. Then however it is not possible to distinguish between a string variable and a filename. McStasScript does however have a dict of all defined parameters, and can test if this would be allowed. The only potential problem is if a variable exists with the same exact name as what is entered, but it was meant as text.

It would be worthwhile to have this system to simplify common tasks.