v.to.db: add a way to control output precision

[jidanni]

v.to.db could add a way to control precision.

$ grass ...TC46 --exec v.to.db -p map=Qingfu_st 
cat|length
1|0.157112134436172

e.g., the user might not be interested in nanometers, etc.

[neteler]

While I am not sure if it is worth it to cut off precision, it could be done in a way as v.out.ascii offers it:

  precision   Number of significant digits (floating point only)
              options: 0-32
              default: 8