Add control structures to cpptraj scripts

[drroe]

This PR adds a control structure framework to cpptraj scripts; specifically for loops. Since it represents a significant feature addition I'm keeping it a work in progress for the time being.

The for loop can currently be over atom masks or integers. Help entry:

	{ {atoms|residues|molecules|molfirstres|mollastres}
	    <var> inmask <mask> parm <name> | parmindex <#> | <#> ... |
	  <var>=<start>;[<var><end OP><end>;]<var><increment OP>[<value>] ... }
	END KEYWORD: 'done'
  Create a 'for' loop around specified mask(s) and/or integer value(s).
  Any number and combination of masks and integers can be specified.
  Variables created in the for loop can be referenced by prefacing
  the name with a '$' character.
  Available 'end OP'       : '<' '>'
  Available 'increment OP' : '++', '--', '+=', '-='

Any number of loop types can be combined into a single for statement. For example:

	for atoms A0 inmask :1-27&!@H= i=1;i++
	  distance d$i :TCS $A0 out $i.dat
	done

Would loop over atoms in mask :1-27&!@H= while setting variable i to 1 and incrementing by 1 each iteration of the loop. If multiple end conditions exist within a single for loop the loop will execute until the first end condition is reached. For loops can also be nested.

Variables created by for loops are called "script" variables and are prefaced by $. This was chosen because it doesn't clash with existing cpptraj syntax and is familiar to almost all programmers. It also won't conflict with existing shell scripts since the $ would need to be protected in order to make it to cpptraj (rendering it a non-variable to the shell script).

NOTE: the for loop could potentially render the commands multicurve and multihist obsolete. The multidihedral and multivector commands have some additional functionality which justifies keeping them around.

This also adds two commands that pertain to "script" variables.

> help set
  [help set]
	<variable>=<value>
  Set script variable <variable> to value <value>.
> help show
  [help show]
  Show all current script variables and their values.

This means that "script" variables can be used as placeholders in cpptraj scripts, e.g.:

set PREFIX = distance
for residues R0 inmask :1-4@O residues R1 inmask :9-12@H i=1;i++
  distance $R0 $R1 out $PREFIX.$i.dat
done

Note that "script" variables in for loops themselves are currently only allowed in the outermost loop; script variables in nested loops are not currently supported.

[drroe]

The jenkins failure is a cuda driver problem I think, not code.

Updated syntax for set:

	{ <variable> <OP> <value> |
	  <variable> <OP> {atoms|residues|molecules} inmask <mask>
	    [parm <name> | parmindex <#> | <#>]
	  <variable> <OP> trajinframes }
  Set (<OP> = '=') or append (<OP> = '+=') a script variable.
  - Set script variable <variable> to value <value>.
  - Set script variable to the number of atoms/residues/molecules in
     the given atom mask.
  - Set script variable to the current number of frames that will
     be read from all previous 'trajin' statements.

Scalar data sets can now be used as script variables as well, although only the first value from a data set will be used. This can be expanded later if necessary.