Fix #101 to support Julia 0.6, 0.7 and 1.0

[emmt]

This PR is both a review and a fix of #101 to extend FITSIO support to all Julia versions ≥ 0.6

The fixes are summarized in the commit comments. The proposed PR passes all the tests with Julia versions 0.6.4, 0.7.0 and 1.0.0.

For the review part, I fully agree with most of the changes done in #101. In fact, they are quite similar to my changes in #103, thus this new PR is a candidate for replacing #101 and #103. In #101 and #103. As discussed in #101 and #103, the most significant changes concern the try_parse_hdrval methods:

• Methods try_parse_hdrval(T,s) (with a type and a string arguments) used to return a Nullable{T} result and now returns either a value of type T or nothing if unsuccessful. This follows the behavior of the tryparse methods in latest Julia versions.

• Method try_parse_hdrval(s) (with a single string argument) was only called by parse_header_val. It has been deleted (in this PR) and its code moved into method parse_header_val.

Note that methods try_parse_hdrval can be assumed to be private as they are not documented in FITSIO doc. and are only called in the file header.jl. These changes should not affect the end users.

Future improvements

• Methods fits_try_read_keys, fits_try_read_extname and fits_try_read_extver may be assumed to be public. Their API shall be clearly documented, defined and fixed. They should be directly tested in runtests.jl.

• The type of the value of a FITS keyword can be inferred from the first non-space character (a quote '\'' must be a string, a letter 'T' or 'F' must be a boolean, only spaces must be an empty value and anything else can only be an integer or a floating-point). This fact can be used to speed-up the parsing of FITS keywords in parse_header_val.

• According to FITS standard Section 5.4.2.6 "Extension Keywords" (https://archive.stsci.edu/fits/fits_standard), EXTNAME shall be a string while EXTVER shall be an integer. Both should not appear in the primary header. This may be taken into account by methods fits_try_read_extname and fits_try_read_extver.

[giordano]

This is great! I hope I'll be able to have a look tonight.

[mweastwood]

Thanks for picking up where I left off! This looks great.

[giordano]

Ok, I think we've waited enough considered that #101 has been open for some months now, I'm going to merge this. Thanks again @emmt and @mweastwood!

[giordano]

@emmt You've outlined some good future improvements: please put them in a new issue, so they won't be lost here :wink:

[emmt]

Yes I will do that. Thanks!