Consistency in "-" and "_" in action and option names

[GiovanniBussi]

I noticed that in action and option names we use a mixture of underscore (_) and dash (-), the latter being used rarely.

You can see all the uses with the command:

grep "^let b:plumedDictionary" $plumedroot/vim/syntax/plumed.vim | grep "\-"

They are:

MULTI-RMSD
PCARMSD SQUARED-ROOT
PROPERTYMAP DEBUG-CLOSE LOG-CLOSE

I would propose to switch to underscore only converting them to

MULTI_RMSD
PCARMSD SQUARED_ROOT
PROPERTYMAP DEBUG_CLOSE LOG_CLOSE

In the transient (or forever), we can allow the dash version as an alias (depending on how much are these keywords used).