Support for default filename in rffit output .dat file

[xmichaelx]

Currently default name for file to be created is suggested but in order to accept it user has to copypaste it. This PR allows for just hitting enter and accepting the default filename. Proposed file name is presented in square brackets - pattern that is commonly used when default value can be provided by just hitting enter.

Allows for following:

rffit -d out.dat -c tle/bulk.tle 
Loaded 10556 orbits

1Save highlighted points, provide filename [type 'default' for 2014-01-01T00:00:00_2202.404_9990_-0001.dat]: default
247 points saved in 2014-01-01T00:00:00_2202.162_9990_-0001.dat

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