scda: Write and read block sections

[tim-griesbach]

scda: Write and read block sections

This PR continues the implementation of the scda file format (cf. #199) by introducing the functionalities for writing and reading block sections. The changes in this PR are in particular

• implementation of fwrite_block and fread_block_data,
• extension of fread_section_header to also handle block section headers,
• internal functions to read and write count entries appearing in section headers,
• a generic internal function to check if parameters are collective,
• extension of the set of internal padding functions to support outputting the padding bytes into a separate buffer, next to the already existing inplace option. This is used in fwrite_block since the block data is allocated by the user and inplace padding would thus require a copy of the block data.
• For the sake of generality: Extension of the calculation of the number of modulo padding bytes in sc_scda_pad_to_mod_len to the case SC_SCDA_PADDING_MOD < 7.