Option to ignore file size check in reading sac files

Checking for consistency between actual and theoretical
file size (from `npts`) can be disabled both in
`SacIO.ReadSacFile()` and in `sac.core.readSAC()`.
This is obtained by passing the parameter: `fsize=False`.

`sac.core.isSAC()` has been rewritten, in order not to
rely on `npts` to check file format, but rather checking
`nvhdr` and `delta`, as in `SacIO.IsSACfile()`.
Testing fsize option in reading sac files

Adds test cases for core.read() and SacIO.ReadSacFile()

See also #549
Making isSAC() more robust
Added `lovrok` to required sac header fields
Some modifications in isSAC()

- nvhdr >=1 (normally it is always 6).
  This avoids validating as sac files those files which
  have a lot of zeros at the beginning.
- lpspol, lovrok and lcalda are allowed to be undefined
  (though this is not good practice for a sac file...)
backwards compatibility for testing
Merge pull request #549 from claudiodsf/sac

Option to ignore file size check in reading sac files

works for me - thanks for fixing - merging ...