Allow floating point values in memory command line options

[ChrisJefferson]

So, I always assumed that -o 2.5G did what I expected (allocated 2.5GB of memory), but turns out the .5 was silently ignored. Switch to using atof so we parse a floating point number.

We still silently ignore badly written constants, but I think this is a useful improvement.

Release notes suggestion:

• Support floating point numbers when specifying how much memory GAP should use, for example "-o 2.5G".

[coveralls]

Coverage decreased (-0.01%) to 85.195% when pulling d57645b on ChrisJefferson:mem-float into 7ea1f43 on gap-system:master.

[fingolfin]

Should we also document this?

[ChrisJefferson]

I felt this was already "documented", in that the docs don't say you can't give a floating point number, and personally I've been writing things like -o 2.5G for ages, not realising the .5 it wasn't doing anything useful.