Prevent saving as invalid matlab variable name

Created by: Famlam

Matlab variable names must start with a letter, then either of letters, numbers or _ (https://www.mathworks.com/help/matlab/matlab_prog/variable-names.html) If the spectrum name doesn't satisfy this (e.g. a spectrum called "01_mysample_1H_echo"), convert it to something that can actually be read by Matlab. (Prefixing nmr_ to ensure the start with a letter, and replacing all invalid chars by _)

Technically, it should also ensure the spectrum name isn't something like if, for, end, ..., but let's ignore that for now as the likelyhood of that is small