Grin: Update mining status code to support multiple "edge_bits" value

[bladedoyle]

Now that we support multiple cuckatoo/cuckaroo sizes having a single "gps" value displayed may be confusing.

The grin-miner tui says "Mining status: Mining at height xxx at yyy GPS

If the user is mining with 2 plugins, a CPU mining edge_bits=29, and a GPU mining edge_bits=30, that message with a single GPS means what?

I see two options (but a third better option may exist):

1) normalize and merge all graph rates into a single GPS
2) display each rate as a separate GPS value

[bladedoyle]

Not exacly an enhancement in itself, these chagnes are needed to complete the enhancement of adding dual pow

[tromp]

I prefer option 2, as option 1 would require endless explanation.