String concatenation

[chipbarnaby]

Add a string concatenation function to the CSE language

str3 = concat( str1, str2)

Useful for e.g. assembling case-specific file names or informative report titles.

A preliminary implementation code appears to work correctly but has not been sufficiently tested or generalized, see PR #

In addition to testing, the following are worth considering:

• Make the function take n arguments (now limited to 2).
• Use a binary operator ("+" or similar) instead of a function.

Also -- add documentation.