Small fixes

[FilipDominec]

Two small fixes and one new feature:

• I could not convince autogen.sh to pass --prefix=/usr/local to configure.sh, which is important to me
• the premature termination of optimisation with 'Finished solve_cw after 1 steps and 0 CG iters' was reported about a year ago; until the newly written code in 1.2.1 works, I suggest to fall back to the norm2() function from MEEP 1.1.1
• band-src-time uses the sine-integral time function to provide well-defined rectangular spectrum (see http://fzu.cz/~dominecf/misc/meep/index.html#bandsource for examples)

[stevengj]

There should really be a configure check for this.

[stevengj]

Please use separate pull requests for different features (i.e. 3 pull requests in this case), so that they can be reviewed, discussed, and merged independently.

I'll close this so that you can resubmit as distinct PRs.

[stevengj]

As noted above, I've fixed 2 of the 3 issues that you mention by recent commits, so please submit a PR just on the new source that you want to provide, so that we can discuss it.