[FEATURE REQUEST] Normalise the way indexes are specified in force files

[lorenzo-rovigatti]

As of now, some forces support using words like "last" or "all" when specifying indexes. However, not all forces do, and those that do not also do not check whether the provided index is an accepted token. As a result, using "last" or "all" instead of a proper index can result in an index of 0, which is clearly not what the user want. Here is an example:

{
type = mutual_trap
particle = 0
ref_particle = last
stiff = 1.
r0 = 5
}

{
type = mutual_trap
particle = last
ref_particle = 0
stiff = 1.
r0 = 5
}

which will generate the following lines in the log file:

INFO: Adding a MutualTrap (stiff=1, stiff_rate=0, r0=5, rate=0, ref_particle=0, PBC=0) on particle 0
INFO: Adding a MutualTrap (stiff=1, stiff_rate=0, r0=5, rate=0, ref_particle=0, PBC=0) on particle 0

Note that the particle and ref_particle fields of both forces have been interpreted as 0.