ENH: Add RPC support

[HaoZeke]

The design is still being threshed out.

[HaoZeke]

Note that, thankfully, there is no real difference between working with this "simpler interface":

@0xbd1f89fa17369103;

# Design kanged from eOn v4 C-style structs
# TODO(rg): Should be more object oriented

struct ForceInput {
 natm   @0 :Int32; # TODO(rg): Do we really need this..
 pos    @1 :List(Float64);
 atmnrs @2 :List(Int32);
 box    @3 :List(Float64);
}

struct PotentialResult {
  energy @0: Float64;
  forces @1: List(Float64);
}

interface CuH2Pot {
  calculate @0 (fip :ForceInput)
    -> (result :PotentialResult);
}

Or this redundant one:

struct Position {
  x @0 :Float64;
  y @1 :Float64;
  z @2 :Float64;
}

struct AtomMatrixRPC {
  positions @0 :List(Position);
}

struct BoxMatrix {
  box @0: List(Vector);
}

struct Vector {
  x @0: Float64;
  y @1: Float64;
  z @2: Float64;
}

struct AtomTypes {
  atomTypes @0: List(Int32);
}

struct PotentialResult {
  energy @0: Float64;
  forces @1: List(ForceVector);
}

struct ForceVector {
  x @0: Float64;
  y @1: Float64;
  z @2: Float64;
}

interface CuH2Pot {
  calculate @0 (positions :AtomMatrixRPC, atomTypes :AtomTypes, boxMatrix :BoxMatrix)
    -> (result :PotentialResult);
}

Which is essentially the design in 0e3d99c and 1dd36ad.

Basically there is no time difference for running two servers.

Command	Mean [ms]	Min [ms]	Max [ms]	Relative
python oclient.py localhost:12345	108.8 ± 12.7	98.9	143.3	1.00
python pyclient.py localhost:12346	109.4 ± 18.3	99.0	182.8	1.01 ± 0.21

Where oclient is the ForceInput variant, and pyclient is the redundant form.

Essentially this seems to suggest that more nested structures are OK, which is better (needs testing of course for buffer size).