Decide "immutability" strategy - viz. Math::Matrix

[librasteve]

Currently only splice() and .data[] = are a Series/DataFrame mutators
So to make these objects read only (ie a soft version of immutable) can be done this way:

1. add a new has Bool $!ro = True; attr
2. add a new override method rw { $!ro = False } (you can manually override ro behaviour twiw)
3. and a new reset method ro { $!ro = False }
4. and a new clone method clone { ... } with :rw adverb
5. error if splice() or data[]= operations are attempted if $!ro [ie. splice is forbidden]
6. check that dyadic operations now work like this c = a.concat(b); [ie. a,b immutable]
7. implement this by psuedo-code:

method concat( \b ) {
    my c = a.clone( :rw );
    c.concat(b);      #the mutating concat we have now (which may contain mutating c.splice(b) operations)
    c.ro;
    c
}

oh and check grep

there are some interesting comments here

maybe concat can just use concat eg.. method concat( @b ) { |self, |@b }

oh and prevent write access to .data (eg. wrapper)