Fix #740: file_prefix argument is inconsistently implemented in fileio.ascii.py

[MohitKambli]

Hello,
Hope you're doing well
As it has been requested in #740, I have eliminated the hard-coded substring fort and injected the file_prefix variable instead to lines 53 and 65 respectively
Kindly let me know if the code changes meet the requirements
Sincere apologies for my mistakes if you find any

Thanks and Regards,
Mohit Kambli

[coveralls]

coverage: 26.759%. remained the same
when pulling cf07f27 on MohitKambli:pyclaw-branch
into f55b884 on clawpack:master.

[MohitKambli]

Hello @mandli @coveralls,
Hope you're doing well
Just so that we're on the same page, this PR has been raised for solving issue #740
On introducing the mentioned code changes, I am able to see the following error in the build logs
AttributeError: module 'clawpack.riemann.static' has no attribute 'fwave'
I tried to figure out the possible root cause of this error, but still not able to figure out
Could you please help me in solving this issue or the possible reason for such error?
It would be a big help from your end

Thanks and Regards,
Mohit Kambli

[mandli]

This is due to a broader change to the Riemann solvers used and requires clawpack/clawpack#259 to be merged in first. It works locally though for me.