Move to new R mappings

@mdpiper has created a new R BMI definition https://github.com/csdms/bmi-r and a new example implementation https://github.com/csdms/bmi-example-r

We can make a choice between:

1. Using new R mappings in grpc4bmi
2. Deprecating R for grpc4bmi (as it depends on rpy2 anyway). Move R models to [RemoteBMI](github.com/eWaterCycle/remotebmi) instead.

Why keep R in grpc4bmi;

- grpc4bmi should be slightly more efficient than RemoteBMI

Why deprecate R in grpc4bmi;

- R models are not high performance themselves anyway, so RemoteBMI overhead doesn't matter much
- Less code doing "the same thing"; less maintenance
- RemoteBMI interfaces with R directly without having to go through `rpy2`