added DEFC method for 1d least squares spline fitting

[jacobwilliams]

• Initial implementation.
• includes a blas module that can be used (can set HAS_BLAS to link to a different one)
• added a unit test.

Fixes #67

[codecov[bot]]

Codecov Report

Merging #74 (1a5c729) into master (42f1401) will decrease coverage by 2.72%. The diff coverage is 60.70%.

Additional details and impacted files

```diff @@ Coverage Diff @@ ## master #74 +/- ## ========================================== - Coverage 76.58% 73.85% -2.73% ========================================== Files 2 4 +2 Lines 2050 2475 +425 ========================================== + Hits 1570 1828 +258 - Misses 480 647 +167 ```