[FEATURE] inherit not solvable property when adding to graph

[dehann]

Subsequent addVariable! addFactor! calls to an element that is solvable=0 should automatically keep that same property.

cc thanks for the suggestion @tonioteran

[dehann]

this is why....

using Distributed # piggy back illustration 
addprocs(4)
using RoME
# and on remote processes
@everywhere using RoME

# grab any old factor graph
fg = generateCanonicalFG_Hexagonal()

# get list of all r"x\d" variables (ie poses in this case)
vsyms = ls(fg, r"x\d") |> sortDFG
# set variable solvable to 0
map(x->setSolvable!(fg, x, 0), vsyms[end-3:end])
# do same for connected factors
map(x->setSolvable!(fg,x,0), union(map(v->ls(fg, v), vsyms[end-3:end])...))

# and solve
tree, smt, hist = solveTree!(fg)

[dehann]

do in the same way as new SolverParams.graphinit