makeIGRF - create 'igrfcoefs.mat' for IGRF

Create 'igrfcoefs.mat' file for using Drew Compston's IGRF modell

Make 'igrfcoefs.mat' file from downloaded igrf modells (in .txt format) for using it in igrf() matlab function:

Modell can be downloaded from here (.txt format):

https://www.ngdc.noaa.gov/IAGA/vmod/igrf_old_models.html

igrf matlab functions (Drew Compston's IGRF modell):

https://www.mathworks.com/matlabcentral/fileexchange/34388-international-geomagnetic-reference-field-igrf-model

Input: file location eg. 'igrf13coeffs.txt'

Output: creates 'igrfcoefs.mat' file in current folder

'igrfcoefs.mat' file should be placed in the folder where igrf will be used