Add mpath7.log filename as an input option in .MPSIM/.MPNAM

[mjigmond]

In the current implementation it's impossible to run multiple tracking simulations in the same directory because the mpath7.log file is hard-coded in MPath7.f90.

[aprovost-usgs]

As noted by @mjigmond, hardwiring of the log file name to "mpath7.log" precluded running more than one simulation in the same directory at the same time, since all the simulations would try to write to the same log file. The log file name can now be optionally specified on the command line, after the simulation file name. If not specified, it defaults to "mpath7.log". (If only one file name is given on the command line, it is interpreted as the simulation file name, since that file is required. If no file names are given, the user is prompted to enter only the simulation file name, since that file is required.) Tested to confirm the ability to run more than one simulation simultaneously in the same directory (provided none of the output files have been given conflicting names). The updated source code has been posted (commit fdb3924), and we're working on a USGS release that includes this update.

[aprovost-usgs]

See relevant updates in commit 20ae10c in response to issue #19.